<PAGE>   1
                                                                     EXHIBIT 4.5



          SECOND AMENDMENT, dated as of August 13, 1998 (this "Amendment"), to
the Second Amended and Restated Credit Agreement, dated as of April 25, 1997
(the "Credit Agreement"), among Sybron International Corporation, a Wisconsin
corporation (the "Parent"), Ormco Corporation, a Delaware corporation
("Ormco"), Kerr Corporation, a Delaware corporation ("Kerr"), Nalge Nunc
International Corporation, a Delaware corporation ("NNI"), Erie Scientific
Company, a Delaware corporation ("Erie"), Barnstead Thermolyne Corporation, a
Delaware corporation ("Barnstead"; Ormco, Kerr, NNI, Erie and Barnstead are
collectively referred to herein as the "Subsidiary Borrowers"), the several
banks and other financial institutions from time to time parties thereto (the
"Lenders"), Chase Securities Inc., as Arranger, and The Chase Manhattan Bank, a
New York banking corporation, as administrative agent for the Lenders
thereunder (in such capacity, the "Administrative Agent").


                              W I T N E S S E T H:


                 WHEREAS, pursuant to the Credit Agreement, the Lenders have
agreed to make, and have made, certain loans and other extensions of credit to
the Parent and the Subsidiary Borrowers; and

                 WHEREAS, the Parent and the Subsidiary Borrowers have
requested, and, upon this Amendment becoming effective, the Lenders have
agreed, that certain provisions of the Credit Agreement be amended in the
manner provided for in this Amendment.

                 NOW, THEREFORE, the parties hereto hereby agree as follows:

         I.      Defined Terms. Terms defined in the Credit Agreement and used
herein shall have the meanings given to them in the Credit Agreement.

         II.     Amendments to Credit Agreement. Section 7 is hereby amended by
deleting paragraph (b) of subsection 7.1 thereof in its entirety and
substituting in lieu thereof the following:

                 "(b) Interest Coverage Ratio. Permit the Interest Coverage
         Ratio for any period of four consecutive fiscal quarters ending during
         any period set forth below to be less than the ratio set forth
         opposite such period below:

<TABLE>
<CAPTION>
                      Date                                   Ratio
                      ----                                   -----
              <S>                                             <C>
              6/30/98 - 6/29/99                               3.25
              6/30/99 - thereafter                            4.00
</TABLE>

         III. Conditions to Effectiveness. This Amendment shall become
effective on the date (the "Amendment Effective Date") on which the Parent, the
Subsidiary Borrowers, the Administrative Agent and the Majority Lenders shall
have executed and delivered to the Administrative Agent this Amendment.
<PAGE>   2


                                                                            2


         IV.     General.

         1.      Representation and Warranties. To induce the Administrative
Agent and the Lenders parties hereto to enter into this Amendment, the Parent
and the Subsidiary Borrowers hereby represent and warrant to the Administrative
Agent and all of the Lenders as of the Amendment Effective Date that the
representations and warranties made by the Loan Parties in the Loan Documents
are true and correct in all material respects on and as of the Amendment
Effective Date, before and after giving effect to the effectiveness of this
Amendment, as if made on and as of the Amendment Effective Date and no Default
or Event of Default shall have occurred and be continuing.

         2. Payment of Expenses. The Parent and the Subsidiary Borrowers agree
to pay or reimburse the Administrative Agent for all of its out-of-pocket costs
and reasonable expenses incurred in connection with this Amendment, any other
documents prepared in connection herewith and the transactions contemplated
hereby, including, without limitation, the reasonable fees and disbursements of
counsel to the Administrative Agent.

         3.      No Other Amendments; Confirmation. Except as expressly
amended, modified and supplemented hereby, the provisions of the Credit
Agreement and the Notes are and shall remain in full force and effect. This
Amendment shall constitute a Loan Document. The Lenders hereby waive any
Default that shall have occurred between June 30, 1998 and the date hereof in
respect of Section 7.1(b).

         4.      Governing Law; Counterparts. (a) This Amendment and the rights
and obligations of the parties hereto shall be governed by, and construed and
interpreted in accordance with, the laws of the State of New York.

         (b)     This Amendment may be executed by one or more of the parties
to this Agreement on any number of separate counterparts, and all of said
counterparts taken together shall be deemed to constitute one and the same
instrument. This Amendment may be delivered by facsimile transmission of the
relevant signature pages hereof.
<PAGE>   3
                                                                               3



         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their respective proper and duly authorized
officers as of the day and year first above written.

                                             SYBRON INTERNATIONAL CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:


                                             ORMCO CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             KERR CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             NALGE NUNC INTERNATIONAL
                                               CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             ERIE SCIENTIFIC COMPANY


                                             By:                  
                                                -------------------------------
                                                Title:

<PAGE>   4
                                                                               4





                                             BARNSTEAD THERMOLYNE CORPORATION



                                             By:                  
                                                -------------------------------
                                                Title:


                                    
                                             THE CHASE MANHATTAN BANK, as     
                                              Administrative Agent and as a   
                                              Lender                          
                                                                              
                                                                              
                                             By:                              
                                                -------------------------------
                                                Title:                       
                                                                             
                                                                             
                                             ABN AMRO BANK N.V.              
                                                                             
                                                                             
                                                                             
                                             By:                             
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             BANK OF AMERICA NATIONAL TRUST AND
                                              SAVINGS ASSOCIATION              
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                             BANK OF MONTREAL                  
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
<PAGE>   5
                                                                           5



                                             BANK ONE, WISCONSIN               
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                                                               
                                             THE BANK OF NOVA SCOTIA           
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             BANK OF SCOTLAND                  
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             NATEXIS BANQUE/BFCE               
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                  


<PAGE>   6
                                                                               6




                                             PARIBAS                           
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             CREDIT AGRICOLE INDOSUEZ          
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             COMERICA BANK                     
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             CREDIT LYONNAIS CHICAGO BRANCH    
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               

<PAGE>   7
                                                                               7


                                        DG BANK DEUTSCHE GENOSSENSCHAFTSBANK   
                                         CAYMAN ISLAND BRANCH                  
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        U.S. BANK NATIONAL ASSOCIATION         
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        FLEET NATIONAL BANK                    
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        THE FUJI BANK, LIMITED                 
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        THE LONG-TERM CREDIT BANK OF JAPAN,    
                                         LTD.                                  
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                       
                                       
                                       


<PAGE>   8
                                                                               8



                                  
                                        MELLON BANK, N.A.                 
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                         
                                        THE BANK OF TOKYO-MITSUBISHI, LTD.,
                                         CHICAGO BRANCH                   
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE MITSUBISHI TRUST AND BANKING  
                                         CORPORATION                      
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE FIRST NATIONAL BANK OF CHICAGO
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SAKURA BANK, LIMITED          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         


<PAGE>   9
                                                                               9



                                        SOCIETE GENERALE                  
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SUMITOMO BANK, LTD. CHICAGO   
                                         BRANCH                           
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE BANK OF NEW YORK              
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SANWA BANK, LIMITED           
                                                                          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                  



<PAGE>   10
                                                                              10


                                        BANQUE NATIONALE DE PARIS CHICAGO 
                                         BRANCH                           
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        BHF BANK AKTIENGESELLSCHAFT GRAND 
                                         CAYMAN BRANCH                    
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        FIRST UNION NATIONAL BANK         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        FIRSTAR BANK MILWAUKEE, N.A.      
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE INDUSTRIAL BANK OF JAPAN,     
                                          LIMITED CHICAGO BRANCH          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                  
                                  



<PAGE>   11
                                                                              11




                                        BANCA COMMERCIALE ITALIANA        
                                          CHICAGO BRANCH                  
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        BANK HAPOALIM, B.M. PHILADELPHIA  
                                         BRANCH                           
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title: