Abstract:
A phenol resin molding compound which comprises phenol resin, a curing agent for the phenol resin, hydrogenated NBR, chopped strands of glass fibers and powdery glass fibers having a length of about 10 to about 200 μm has a considerably improved elongation and an elevated safely factor from cracking without any substantial lowering of the modulus of tensile elasticity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a phenol resin molding compound, and more particularly to a molding compound comprising phenol resin and fibrous materials. 
     2. Related Prior Art 
     It is the conventional practice to mix phenol resin as a molding material with chopped strands of such fibrous materials as glass fibers, carbon fibers, aramid fibers or the like, cut to a length of 1.5 to 6 mm, thereby improving the strength of molded articles. 
     Molded articles can be produced by injection molding, transfer molding or the like, for example, by melting a compound and passing the molten compound through a mold cavity. When there is an obstacle such as a pin, an insert or the like in the mold cavity, or when the molding is carried out by multigating, the molten resin behaves in such manner that one flowing molten resin stream is separated into a plurality of substreams at the forward end of the stream once and then joined together, and thus occurrence of weld lines is inevitable. 
     The strength along the weld lines formed by molding the conventional compound, particularly the strength against the tensile stress in the direction perpendicular to the weld lines, is very low. Phenol resin compounds containing chopped strands are not exceptional. 
     The present applicants proposed a phenol resin molding compound comprising phenol resin, a curing agent for the phenol resin, chopped strands of glass fibers and powdery glass fibers having a length of about 10 to about 200 μm as a phenol resin molding compound containing chopped strands of glass fibers for improving the weld strength of molding products (U.S. Pat. No. 5,559,179). 
     The proposed phenol resin molding compound satisfied the desired improvement of the weld strength of molding products, but further improvement of elongation has been desired. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a phenol resin molding compound with improved elongation, which comprises phenol resin, a curing agent for the phenol resin, chopped strands of glass fibers and powdery glass fibers having a length of about 10 to about 200 μm. 
     According to the present invention, the phenol resin molding compound further contains hydrogenated NBR. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Phenol resin for use in the present invention includes novolak resin and resol resin, where novolak resin is preferable. Novolak resin having a weight average molecular weight of about 1,000 to about 5,000 is particularly preferable. Below about 1,000, the flowability is so high that there is a fear of generation of a large amount of burrs or occurrence of burn marks due to gas generation during molding, whereas above about 5,000 the flowability is so low that molding is generally hard to carry out. 
     A curing agent such as hexamethylenetetramine, etc. is added to the phenol resin. That is, preferably about 10 to about 30 parts by weight of the curing agent is added to 100 parts by weight of the phenol resin. 
     Hydrogenated NBR for use in the present invention contains 36 to 44% by weight of combined acrylonitrile and has an iodine number of less than 28 g/100 g, preferably 4 to 28 g/100 g. When hydrogenated NBR having a larger degree of unsaturation, that is, a larger iodine number showing a degree of hydrogenation greater than 28 g/100 g, the heat resistance will be lowered. Hydrogenated NBR having an iodine number less than 4 is not commercially available and also is not economical from a viewpoint of cost. About 1 to about 15 parts by weight, preferably about 2 to about 10 parts by weight, of hydrogenated NBR is used per 100 parts by weight of phenol resin. Below about 1 parts by weight the improvement of elongation as desired in the present invention will not be attained, whereas above about 15 parts by weight the modulus of elasticity will be abruptly lowered. 
     Chopped strands of glass fibers for use in the present invention are strands each of about 1,000 to about 30,000 of glass fibers having a fiber diameter of 6 to 30 μm, bundled by a binder such as epoxy resin, vinyl acetate resin, etc. and cut to a length of 1.5 to 6 mm. About 5 to about 150 parts by weight, preferably about 40 to about 110 parts by weight, of the chopped strands of glass fibers are used per 100 parts by weight of the phenol resin in the same manner as so far used. 
     Powdery glass fibers having a length of about 10 to about 200 μm for use in the present invention are those prepared by grinding glass fibers having a fiber diameter of 6 to 30 μm to the specific length of about 10 to about 200 μm in a grinder or the like. About 50 to about 200 parts by weight, preferably about 60 to about 160 parts by weight, of the powdery glass fibers are used per 100 parts by weight of the phenol resin. When the length of the powdery glass fibers is less than about 10 μm, the overall strength will be lowered, whereas above about 200 μm the strength in the weld regions will not be improved. Below about 50 parts by weight the desired improvement of the strength in the weld regions will not be obtained, whereas above about 200 parts by weight the blendability is deteriorated, though the strength in the weld regious can be increased. 
     It is possible to use chopped strands and powder of carbon fibers, aramid fibers, etc. in place of the chopped strands and powder of the glass fibers, but the chopped strands and powder of glass fibers are preferable from the viewpoints of strength of molded articles and cost. Chopped strands and powders of carbon fibers, aramid fibers, etc. can be used together with those of glass fibers, so far as there is no adverse effect. 
     Besides the foregoing components it is also possible to use a silane-based, titanate-based or aluminum-based coupling agent for improving the bonding between the phenol resin and the glass fibers, a coloring agent, a curing promoter, a mold release agent, etc. when desired. A preferable coupling agent is an aminosilane-based coupling agent. Generally, about 0.1 to about 2 parts by weight of a coupling agent is used per 100 parts by weight of glass fibers. 
     The present molding compound is prepared by kneading phenol resin and hydrogenated NBR at first, then pulverizing the kneaded product, thereby obtaining a preliminary kneaded mixture, and then further kneading the preliminary kneaded mixture with other compound components, followed by cooling and pulverization by a mill-type pulverizer, etc. The thus prepared molding compound is then subjected to injection molding, transfer molding, etc. 
     By adding powdery glass fibers having a length of about 10 to about 200 μm to a phenol resin molding compound comprising phenol resin and chopped strands of glass fibers, the strength of weld regions of molded articles produced therefrom by injection molding, transfer molding, etc. can be improved. As a result, molded articles having weld regions produced from the present phenol resin compound have a distinguished durability even if used in a high load state or an atmosphere subject to a thermal stress. Furthermore, molded articles having weld regions inserted with a metal or the like, such as stators made from phenol resin for use in automatic speed regulators, obtained from the present phenol resin molding compound, have a good resistance to cracking. 
     By further adding hydrogenated NBR thereto, the elongation of weld regions can be considerable improved without lowering the modulus of elasticity. Thus, vulcanized molded articles obtained from the present phenol resin molding compound have a smaller strain at stress loading due to the higher modulus of elasticity and also a larger allowable strain due to the higher elongation, and consequently crackings (breakage) can be effectively prevented. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     The present invention will be described in detail below, referring to Examples and Comparative Examples. 
     EXAMPLES 1 TO 5 
     The following components were used to prepare phenol resin molding compounds according to the present invention: 
     
         ______________________________________            Parts by weight______________________________________Novolak phenol resin*.sup.1              100Hexamethylenetetramine              25Hydrogenated NBR**.sup.2              2        (for Example)              4        (for Example)              6        (for Example)              10       (for Example)              15       (for Example)Chopped strands of glass fibers              150(fiber diameter: 13 μm)Powdery glass fibers              75(fiber diameter: 11 μm;average fiber longth: 70 μm)Stearic acid       4Calcium hydroxide  5Aminosilane-based coupling agent              4______________________________________ Note: *.sup.1 PR29274H, a product of SumitomoDurets K. K., Japan **.sup.2 Zetpol 200L, a product of Nihon Zeon K. K., Japan 
    
     The thus obtained phenol resin compounds were formed into test pieces in the shape according to ASTM D-638, Type I by injection molding at a molding condition of a cylinder temperature of 90° C., a mold temperature 175° C., an injection pressure of 800 kgf/cm 2 , an injection time of 30 seconds and a curing time of 30 seconds. 
     Test pieces for determining modulus of tensile elasticity and elongation in the weld region were prepared by introducing the molten compound from both ends of the mold. Drawing tests for determining the tensile strength were carried out according to ASTM D-638. 
     Results of determination are given in Table 1. 
     
                       TABLE 1______________________________________      Modulus of tensile elasticity                       ElongationExample No.      (kgf/mm.sup.2)   (%)______________________________________1          2120             0.892          2080             0.913          2010             0.914          1900             0.885          1770             0.75______________________________________ 
    
     Comparative Example 1 
     No hydrogenated NBR was used in Example 3. 
     Comparative Example 2 
     No powdery glass fibers were used in Example 3. 
     Comparative Example 3 
     Neither hydrogenated NBR nor powdery glass fibers were used in Example 3. 
     Modulus of tensile elasticity and elongation in the weld regions of injection molded test pieces obtained from the phenol resin molding compounds of Comparative Examples 1 to 3 were determined in the same manner as in Examples 1 to 5. The results are shown in Table 2. 
     
                       TABLE 2______________________________________       Modulus of tensile elasticity                        ElongationCom. Ex. No.       (kgf/mm.sup.2)   (%)______________________________________1           2120             0.722           1700             0.803           2330             0.37______________________________________ 
    
     Safety factors (maximum load expecting in using molded product) from crackings in molded test piece models were calculated and found to be 1.7 for Example 1 and 1.3 for Comparative Example 1, but cracking for Comparative Example 3.