Abstract:
A method for enhancing surface properties of a polymeric solid is disclosed involving chlorination in the presence of actinic light to a depth substantially not in excess of 1800 Å and to a chlorine content by weight in said 1800 Å layer not exceeding 20%. Such limited chlorination of only the molecular surface layer provides enhancement of the polymeric solids&#39; surface and barrier properties without any perceptible deterioration of the polymer&#39;s bulk physical properties.

Description:
FIELD OF INVENTION 
     This invention relates to surface modification of polymeric solids. More particularly, this invention discloses the discovery that chlorination of the molecular surface layer of polymeric surfaces, especially transparent surfaces, involves a discontinuous phenomena. 
     DISCLOSURE OF RELATED ART 
     Chlorination of polymeric solids such as polyolefins is known in the art for purposes of minimizing tackiness attributable to plasticizer migration. Chlorination is also known in the art for purposes of hardening polymeric solids and making such solids UV light resistant through conversion of the entire surface to chloro-saturated polymers. 
     U.S. Pat. No. 2,276,951 discloses treating polyisobutylene with actinic light such that 20% to 50% chlorine content by weight is obtained throughout a surface thickness of 0.01 inch. 
     The present invention advances the state of the art based upon the discovery that the improvement in surface properties of polymeric solids is a discontinuous nonlinear phenomena and that above a threshold value actual harmful surface degradation or modification occurs. Applicants have additionally discovered that chlorination to 1800Å depth (one ten-billion th  inch), or in other words chlorination of only the molecular surface layer, provides optimum surface improvement. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The inventor has discovered that enhancement of surface properties through surface chlorination of polymeric solids is a discontinuous phenonema. Surface properties are enhanced up to a fine threshold level of chlorine concentration. Above the threshhold level abrupt nonlinear deleterious changes in surface properties occur. Properties such as solvent resistance reflected in solvent contact angles abruptly deteriorate. Haze and flex crazing drastically increase above the threshold level. The rate of increase in density abruptly accelerates above the threshhold level. 
     The present invention is the discovery of peak enhancement of surface properties by means of a mild surface etch of only the molecular surface layer of polymeric solids. The surprising aspect of the discovered threshhold level is that it lies at least four magnitudes of order lower than taught by the prior art. For example, Fisher (2,276,951) teaches chlorination to a depth of 0.01 inch and a chlorine concentration of 20-50% by weight. 
     The present invention discloses surprisingly that surface properties of polymeric solids are enhanced when chlorinated to a depth not to exceed 1800 Angstroms and a chlorine concentration in the 1800 Angstroms layer of less than 20%. (1Å=3.937×10 -9  inch) Above this level of concentration and depth of penetration surface properties deteriorate. Such deterioration is particularly severe for transparent polymers. 
     Gaseous chlorination of solid polymers yields a polymer having a chlorinated surface layer in which the chlorination concentration gradient decreases with penetration or depth into the polymeric solid. 
     The prior art teaches chlorine penetration to 0.01&#34; for optimum properties. This involves penetration 10,000×further into the polymeric solid than taught by the present invention. Penetration to such depth as taught in the prior art involves chloro-saturation of the surface molecular layer. This saturation chlorination irreparably damages the molecular surface layer of the polymeric solid. 
     Density, for example, is a major parameter for consideration in the photochlorination modification of polymer molecular surface layers. Table 1 illustrates the relationship between the physical properties of chlorinated polymers and their densities. Table 1 demonstrates simply that changes in physical properties occur when a significant chlorine content is imparted to a polymer. 
     
                                           TABLE 1__________________________________________________________________________Relationship Between Physical Properties of ChlorinatedPolymers and Their Densities*            Tensile       Dielectric            Modulus       Strength            ASTM D-638                   % Elongation                          [Vcm.sup.-1 ] · 10.sup.-3                                  Permeability% Cl    Polymer Density            [MPa] · 10.sup.-3                   ASTM D-638                          ASTM D-149                                  PH.sub.2 O__________________________________________________________________________ 0% Polyethylene       0.85 to            0.1 to 1.2                   800-900                          180-390 low       1.0057% Chlorinated       1.38 to            2.4 to 4.1                    2-40  160-590 high    Polyethylene,       1.52    i.e. Polyvinyl    chloride (PVC)73% Chlorinated       1.65 to            0.34 to 0.55                   250    160-240 low    Polyethylene,       1.95    i.e. Polyvinyl-    idene chloride    (PVDC)__________________________________________________________________________ *Information obtained from Polymer Handbook, Second Edition, J. Brandrup and E. H. Immergent, editor, J. Wiley, 1975. 
    
     Table 2 lists relative density information concerning chlorinated hydrocarbons. As Table 2 illustrates, the density of chlorinated materials is not a linear function of the % chlorine content in the molecular structure from 0 to 80% chlorine by weight. From 0 to approximately 36.65% Cl the slope of a line relating % Cl versus the density of chlorinated hydrocarbons minus the density of nonchlorinated parent hydrocarbon (dCl-d o ) is 1.19 while the slope of a line between 45 and 75% chlorine (dCl-d o ) is 0.66. Hence physical property correlations or expected performance characteristics cannot be simply linearly extrapolated from high to low or low to high levels of chlorine on a molecular surface. 
     
                                           TABLE 2__________________________________________________________________________Densities and % Cl of hydrocarbon materials                 Density of non-                 chlorinatedStructure Structure       parent hydro-Chlorinated Unchlorinated   carbonCompound Compound         % Cl             dCl do      Δd = dCl - do__________________________________________________________________________C.sub.18 H.sub.37 Cl --      12  0.849       0.072 C.sub.18 H.sub.38          0      0.777C.sub.14 H.sub.29 Cl         15   0.8589      0.0959 C.sub.14 H.sub.30          0      0.763C.sub.10 H.sub.21 Cl         20  0.868       0.138 C.sub.10 H.sub.22          0      0.730C.sub.10 H.sub.20 Cl.sub.2         34  0.999       0.269C.sub.8 H.sub.15 Cl         24  0.875       0.172 C.sub.8 H.sub.18          0      0.703C.sub.7 H.sub.15 Cl         26  0.881       0.197 C.sub.7 H.sub.16          0      0.684C.sub.7 H.sub.14 Cl.sub.2         42  1.064       0.38C.sub.6 H.sub.13 Cl         29  0.879       0.22 C.sub.6 H.sub.16          0      0.659C.sub.6 H.sub.12 Cl.sub.2         46  1.068       0.409C.sub.5 H.sub.11 Cl         33  0.882       0.256 C.sub.5 H.sub.12          0      0.626C.sub.5 H.sub.10 Cl.sub.2         50  1.106       0.48C.sub.5 H.sub.9 Cl.sub.3         61  1.253       0.63C.sub.6 H.sub.10 Cl.sub.4         63  1.32        0.661 C.sub.6 H.sub.14          0      0.659C.sub.6 H.sub.9 Cl.sub.5         69  1.37        0.771__________________________________________________________________________ 
    
     Chlorination to only 1800Å depth and not exceeding 20% chlorine by weight i.e. chlorination of only the molecular surface layer provides polymeric solids having enhanced solvent resistance, dirt resistance, and resistance to penetration by highly toxic agents such as chemical warfare agents. Chlorination of a polymeric solid to achieve such achievements in barrier properties without perceptibly deteriorating any of the bulk physical properties of the polymeric solid is highly desirable. 
     The process of the present invention successfully produces chlorine groups only on the surface of a polymer in relatively low concentrations (see Table 3). 
     
                       TABLE 3______________________________________ESCA Data of Chlorinated Polymeric MaterialsPolymer       Average % ofSubstrate     Chlorination (1200Å)______________________________________polyethylene  13%Butyl rubber  17%EPDM           6%polycarbonate  5%nitrile rubber         10%neoprene      12%Fire hose      4%______________________________________ 
    
     The % Cl 2  in the reaction feed mixture causes chlorine functional groups to be formed on the polymer surface in a non-linear manner (Table 4). Both the % Cl 2  in the reaction feed mixture and the actual % Cl on the polymer surface cause a non-linear change in density for the modified polymer system (Table 4). 
     
                       TABLE 4______________________________________       % Cl.sub.2 in the                      Change in       reaction gas   polymer% Cl on the feed mixture   (polyethylene)polymer surface       (30 min. reaction time)                      density______________________________________0            0             1.1872-3         10             1.18823-4         40             1.18805-6         50             1.18945-7         65             1.1935-7         100            1.20______________________________________ Effect of Chlorine (reaction mixture and polymer surface) on the change i density of a polyethylene sample. 
    
     Changes in Physical Properties 
     The changes in % haze and contact angle for surface chlorinated polypropylene, ethylene propylene rubber and polyethylene as a function of % Cl 2  in the reaction feed mixture are shown in Tables 5 and 6. 
     
                       TABLE 5______________________________________% Cl.sub.2 inthe reaction   % hazemixture feed   polypropylene and(30 min. reaction time)          ethylene propylene rubber______________________________________ 0             10-1210             14-1550             15-1860             1685             18100            20______________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________(a)     (b)           (c)           (d)          (e)% Cl.sub.2 in the   Contact angle for 5%/45%                 Contact angle for                               Contact angle for                                            Contact angle forreaction feed   isopropyl alcohol IPA/H.sub.2 O                 10%/90% isopropyl alcohol                               25%/75% isopropyl alcohol                                            35%/65% isopropyl                                            alcoholgas mixture   ∠θ°                 IPA/H.sub.2 O IPA/H.sub.2 O                                            IPA/H.sub.2 O__________________________________________________________________________  0%    76            70            60           4510      65            55            32           1950      72            58            34           17100     70            57            32           14__________________________________________________________________________ 
    
     Table 6 is particularly instructive of the phenomena of the present invention. Note that in any individual column b, c, d or e, that after 10% chlorine when higher levels of chlorine are used in the feed mixture the contact angle deteriorates following initial chlorine exposure. Nothing is gained with additional chlorine exposure and in fact harmful degradation occurs. Table 7 further demonstrates this harmful degradation when further exposure occurs. Note in previous Table 6 that in row 2 the contact angle changed as compared to row 1. In other words, the surface energies changed. Further exposure (rows 3 and 4) caused no further appreciable change in the surface properties. 
     
                       TABLE 7______________________________________                            HD--CW     Cl.sub.2 Concentration AgentSubstrate (% in N.sub.2)                  Properties                            Permeation______________________________________Neoprene  10 to 50%    Flexible  NoneRubber GloveNeoprene   50 to 100%  Cracked   --Rubber GloveNeoprene  0            Flexible  HD PermeableRubber Glove______________________________________ HD--CW is a mustard gas (dichloroethylene sulfide  Cl--CH.sub.2 --CH.sub. --S--CH.sub.2 --CH.sub.2 --Cl) reaction time 30 min. (2 lamps) 
    
     
                       TABLE 8______________________________________           Cl.sub.2 ConcentrationSubstrate       (% in N.sub.2)                        % Haze______________________________________EPDM (clear) rubber            0           6.9           14           7.0           50           8.0 to 14           100          18.4.sup.(a)______________________________________ .sup.(a) exhibited severe hazing and cracking upon flexing reaction time 30 min. (2 lamps) 
    
     Table 8 demonstrates further that exposure to gaseous chlorine in conentrations and exposures exceeding 14% for 30 minute time periods results in increased hazing and thus supports the invention that chlorination of only the molecular surface layer is desirable. 
     The present process improves over prior art processes by teaching that the molecular surface layer characteristics of a polymeric solid, particularly transparent solids and solids such as polyalkenes, butyl rubbers, polycarbonates, nitrile rubbers and neoprenes, are enhanced when the polymeric solid is chlorinated by gaseous chlorine in the presence of actinic light such that chlorination does not exceed a depth of 1800Å and a chlorine content not above 20% by weight. 
     It is possible to practice the invention by pre-irradiating the chlorine gas with actinic light before contacting the surface of the polymeric solid; however, simultaneous exposure is preferred. 
     While the invention has been described with respect to preferred embodiments, other equivalents will be apparent to those skilled in the art, thus, it is intended that the invention be limited only by the scope of the appended claims.