Abstract:
A method is provided for treating the outer surfaces of a plurality of glass bubbles. That method includes loading a plurality of glass bubbles into a processing vessel having a roughened lining and displacing the processing vessel so that the plurality of glass bubbles move against the roughened lining to thereby roughen the outer surfaces. Alternatively, or in addition, the glass bubbles are subjected to air plasma treatment to increase the surface energy of the glass bubbles.

Description:
TECHNICAL FIELD 
       [0001]    This document relates generally to a surface treatment method for glass bubbles, glass bubbles manufactured by that method and low density sheet molding compound made utilizing glass bubbles manufactured by that method. 
       BACKGROUND 
       [0002]    The government&#39;s corporate average fuel economy (CAFE) requirement of 54.5 mpg by 2025 has pushed motor vehicle manufacturers to accelerate the use of lightweight materials in motor vehicles. As a result, it has been proposed to utilize low density sheet molding compounds in the construction of vehicle body panels, such as the hood, instead of standard density sheet molding compounds. More specifically, low density sheet molding compounds have a density of about 1.2 g/cm 3  while standard density sheet molding compounds have a density of about 1.9 g/cm 3 . The key weight reduction in the formulation of low density sheet molding compounds versus standard density sheet molding compounds is replacing the CaCO 3  component of the standard sheet molding compounds with low density glass bubbles. 
         [0003]    One of the technical challenges faced in this effort is the low interfacial properties between glass bubbles and resin. More specifically, the smooth/hard surface of the glass bubbles causes weak bonding of the bubbles to the resin matrix which has a tendency to substantially decrease the overall mechanical performance of low density sheet molding compound. 
         [0004]    This document relates to a new and improved surface treatment method for glass bubbles which functions to roughen the surface of the glass bubbles so as to allow for better adhesion to the resin. 
       SUMMARY 
       [0005]    In accordance with the purposes and benefits described herein, a method is provided for treating the outer surfaces of a plurality of glass bubbles. That method may be broadly described as comprising the steps of: (a) loading a plurality of glass bubbles into a processing vessel having a roughened lining and (b) displacing the processing vessel so that the plurality of glass bubbles move against the roughened lining and thereby roughen the outer surfaces. 
         [0006]    In one possible embodiment, the method includes spinning and rotating the processing vessel. More specifically, the method may include spinning the processing vessel at a speed of between about 60 rpm and about 600 rpm and rotating the processing vessel at a speed of between about 60 rpm and about 600 rmp where the spinning and rotating are provided along two different axes. The method also includes reversing the direction of the spinning and the rotating of the processing vessel. In one possible embodiment, the processing vessel has a spherical shape. In another possible embodiment, the processing vessel has a cylindrical shape. 
         [0007]    Still further, the method may include providing the plurality of glass bubbles with an outer surface roughness of about 0.01% to about 0.1% of the diameter of the plurality of glass bubbles. Still further, in one possible embodiment, the method may include subjecting the plurality of glass bubbles to an air plasma treatment. This may be done by moving the plurality of glass bubbles through an air plasma stream using a vibrating conveyor belt, moving the plurality of glass bubbles through an air plasma stream on a slide or dropping the plurality of glass bubbles through an air plasma stream. In any of these embodiments, the method includes subjecting the plurality of glass bubbles to an air plasma temperature of between about 23° C. and about 500° C. 
         [0008]    In one possible embodiment, the air plasma treatment is completed after treating the plurality of glass bubbles in the processing vessel. In one possible embodiment, the glass bubbles are only treated mechanically in the processing vessel. In another possible embodiment, the glass bubbles only receive an air plasma treatment. 
         [0009]    In accordance with an additional aspect, a method is provided for treating the outer surfaces of a plurality of glass bubbles comprising the step of subjecting the plurality of glass bubbles to an air plasma treatment so as to provide an outer surface roughness of about 0.01% to about 0.1% of the diameter of the plurality of glass bubbles. 
         [0010]    In accordance with yet another aspect, a glass bubble is provided wherein that glass bubble comprises a hollow glass body having an outer surface of a diameter of between about 16 μm and about 25 μm and a surface roughness of about 0.01% to about 0.1% of that diameter. Still further, a low density sheet molding compound is provided comprising a resin and a plurality of glass bubbles as set forth and just described. 
         [0011]    In the following description, there are shown and described several preferred embodiments of the method and the glass bubble product of the method. As it should be realized, the method and product are capable of other, different embodiments and their several details are capable of modification in various, obvious aspects all without departing from the method and product as set forth and described in the following claims. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0012]    The accompanying drawing figures incorporated herein and forming a part of the specification, illustrate several aspects of the method and product of the method and together with the description serve to explain certain principles thereof. In the drawing figures: 
           [0013]      FIGS. 1 a  and 1 b    are schematic representations illustrating a glass bubble with an original smooth surface and the same glass bubble with a roughened surface after undergoing the method of surface treatment described herein. 
           [0014]      FIG. 2 a    is a schematic illustration of one possible embodiment of a device for subjecting glass bubbles in a spherical container with a roughened inner surface to rotation and spinning along two different axes A 1 , A 2 . 
           [0015]      FIG. 2 b    is a view similar to  FIG. 2 a    but showing an alternative embodiment wherein the processing vessel is cylindrical in shape. 
           [0016]      FIG. 3 a    is a schematic illustration of one possible way to subject a plurality of glass bubbles to an air plasma treatment utilizing a vibrating conveyor belt. 
           [0017]      FIG. 3 b    is a schematic view illustrating an alternative embodiment wherein the glass bubbles are subjected to air plasma treatment while rolling down a ramp or a slide. 
           [0018]      FIG. 3 c    is a schematic illustration of another embodiment wherein a plurality of glass bubbles subjected to an air plasma treatment by dropping and falling through a tube. 
       
    
    
       [0019]    Reference will now be made in detail to the present preferred embodiments of the method, examples of which are illustrated in the accompanying drawing figures. 
       DETAILED DESCRIPTION 
       [0020]    Reference is now made to  FIG. 1 a    illustrating a single glass bubble  10  comprising a hollow body  12  and a smooth outer surface  14 . As shown in  FIG. 1 b   , after being subjected to the method of treating that outer surface, the glass bubble  10  includes a hollow outer body  12  with a roughened outer surface  20 . In one useful embodiment, the treated glass bubble  10  has a diameter of between about 16 μm and about 25 μm and a surface roughness of about 0.01% to about 0.1% of that diameter: that is, a surface roughness of between 160 nm to 2500 nm. A plurality of surface treated glass bubbles  10  as disclosed have a density of about 1.2 g/cm 3 . Further, the roughened outer surface  20  has the desired interfacial interaction with a resin to provide a low density sheet molding compound with desired mechanical properties and performance when used as a motor vehicle panel component. 
         [0021]    In order to achieve the desired roughening of the glass bubble surface  20 , the glass bubbles may be subject to roughening in a spinning and rotating processing vessel as illustrated in  FIGS. 2 a  and 2 b   , or air plasma treatment as illustrated in  FIGS. 3 a -3 c    or both. 
         [0022]      FIG. 2 a    illustrates one possible embodiment of a device  30  for processing the glass bubbles  10 . The device  30  includes a processing vessel  32  of spherical shape that has a roughened internal lining  34 . The processing vessel  32  is supported at two opposed points by means of the yoke  36 . A drive motor  38  rotates (see action arrow B) the yoke  36  and, therefore, the processing vessel  32  about a first axis A 1 . A second drive motor  40  is carried on the yoke  36  and functions to spin the processing vessel  32  about a second axis A 2  (see action arrow C). In the illustrated embodiment, the first axis A 1  and the second axis A 2  are substantially perpendicular to each other. 
         [0023]    A controller  42  connected to the two drive motors  38 ,  40  allows the speed of the drive motors to be set and changed as desired during the processing of the glass bubbles  10 . The goal is to ensure that the entire outer surface of the plurality of glass bubbles  10  are roughened consistently throughout so as to provide an outer surface roughness of about 0.01% to about 0.1% of a diameter of the plurality of glass bubbles. Toward this end, the processing vessel  32  may be spun about axis A 1  at a speed of between about 60 rpm and 600 rpm while being rotated about axis A 2  at a speed of between about 60 rpm and 600 rpm. The controller  42  also allows the direction of spinning and the direction of rotating of the processing vessel  32  to be reversed as desired. Thus, for example, the processing vessel  32  may be rotated by the drive motor  38  in a first direction at a speed of between 60 and 600 rpm for a first period of time of, for example, from five to sixty seconds and then rotated in a second direction at a speed of between 60 and 600 rpm for a second period of time of, for example, from five to sixty seconds. Similarly, the processing vessel  32  may be spun by the drive motor  40  in a third direction at a speed of between 60 and 600 rpm for third period of time of, for example, from five to sixty seconds and then spun in a fourth direction at a speed of between 60 and 600 rpm for fourth period of time of, for example, from five to sixty seconds. The rotating and spinning and the periods of time may be sequential or overlapping. The reversing of direction aids in ensuring that the outer surface  20  of the glass bubbles  10  receive consistent overall roughening. 
         [0024]      FIG. 2 b    illustrates a device  46  similar to the device  30  illustrated in  FIG. 2 a   . The device  46  includes a processing vessel  48  having a roughened lining  50 , a yoke  52 , a first drive motor  54 , a second drive motor  56  and a controller  58 . The two devices  30 ,  46  operate identically, the only difference is the processing vessel  48  in the second device  46  is cylindrical in shape rather than spherical in shape. 
         [0025]    Reference is now made to  FIGS. 3 a -3 c    illustrating three different devices for providing an air plasma treatment to the glass bubbles  10 . In the embodiment illustrated in  FIG. 3 a   , the glass bubbles  10  are positioned on a face  60 , of a vibrating conveyor belt  62 . Accordingly, the glass bubbles  10  bounce up and down as they move along the vibrating conveyor belt  62  through an air plasma stream  64  emanating from the overlying plasma probe  66 . 
         [0026]    As illustrative  FIG. 3 b   , the glass bubbles  10  roll down an incline, slide or ramp  68  through a plasma stream  70  emanating from the overlying plasma probe  72 . 
         [0027]    In the embodiment illustrated in  FIG. 3 c   , the glass bubbles  10  fall or drop through the tube  74  falling through the air plasma stream  76  emanating from the opposed plasma probes  78 . In any of the air plasma treatment embodiments illustrated in  FIGS. 3 a -3 c   , the glass bubbles  10  are subjected to air plasma temperatures of between about 23° C. and about 500° C. and undergo an increase in surface energy that enhances their chemical bonding to a resin such as used in the production of a sheet molding compound. This increases the overall mechanical performance of a part made from that low density sheet molding compound thereby making the parts useful as panels such as hoods or the like in a motor vehicle. 
         [0028]    With the unique size and shape of glass spheres, the above three treatment processes are specially designed to allow for a full surface treatment. The bouncing, sliding and falling movements ensure that the glass bubbles have full exposure to the plasma stream. A full surface treatment imparts better mechanical properties and performance to the glass bubbles than a partial treatment. 
         [0029]    In summary, numerous benefits result from the surface treatment method disclosed in this document whether that method comprises only processing the glass bubbles  10  in the spinning and rotating processing vessels  32 ,  48  illustrated in  FIGS. 2 a  and 2 b   , only processing the glass bubbles by means of the air plasma treatments illustrated in  FIGS. 3 a -3 c    or subjecting the glass bubbles to both rotating and spinning in the processing vessel and air plasma treatment. 
         [0030]    By increasing the roughness and/or surface energy of the surface  20  of the treated glass bubbles  10 , chemical bonding between the glass bubbles and any resin used to make low density sheet molding compounds is enhanced thereby providing those sheet molding compounds with superior mechanical properties allowing their use as various panel components of a motor vehicle. This advantageously allows the production of lower weight motor vehicles which are characterized by increased fuel economy. Accordingly, the method disclosed herein and the resulting glass bubbles and low density sheet molding compound products made using the glass bubbles  10  with the roughened surfaces  20  represents a significant advance in the art. 
         [0031]    The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.