<PAGE>

                                                                    Exhibit 4.15

                          CERTIFICATE OF INCORPORATION

                                       OF

                                MED RESINS, INC.

                                   ----------

     The undersigned, a natural person, for the purpose of organizing a
corporation for conducting the business and promoting the purposes hereinafter
stated, under the provisions and subject to the requirements of the laws of the
State of Delaware (particularly Chapter 1, Title 8 of the Delaware Code and the
acts amendatory thereof and supplemental thereto, and known, identified, and
referred to as the "General Corporation Law of the State of Delaware"), hereby
certifies that:

     FIRST: The name of the corporation (hereinafter called the "Corporation")
is MED Resins, Inc.

     SECOND: The address, including street, number, city, and county, of the
registered office of the Corporation in the State of Delaware is 1013 Centre
Road, Wilmington, DE 19805, County of New Castle; and the name of the registered
agent of the Corporation in the State of Delaware at such address is Corporation
Service Company.

     THIRD: The nature of the business or purposes to be conducted by and
promoted by the Corporation is to conduct any lawful business, to promote any
lawful purpose, and to engage in any lawful act or activity for which
corporations may be organized under the General Corporation Law of the State of
Delaware.

     FOURTH: The total number of shares of stock which the Corporation shall
have authority to issue is three thousand (3,000) shares of Common Stock, par
value one cent ($.01) per share.

     FIFTH: No Director of the Corporation shall be personally liable to the
Corporation or its stockholders for monetary damages for breach of fiduciary
duty as a Director; provided, however, that the foregoing clause shall not apply
to any liability of a Director (i) for any breach of the Director's duty of
loyalty to the Corporation or its stockholders, (ii) for acts or omissions not
in good faith or which involve intentional misconduct or a knowing violation of
law, (iii) under Section 174 of the General Corporation Law of Delaware, or (iv)
for any transaction from which the Director derived an improper personal
benefit. This Article shall not eliminate or limit the liability of a Director
for any act or omission occurring prior to the time this Article became
effective.

<PAGE>

     SIXTH: The name and mailing address of the Incorporator is as follows:

NAME:                                   MAILING ADDRESS:

Douglas G. Gray, Esq.                   c/o Edwards & Angell, LLP
                                        2800 BankBoston Plaza
                                        Providence, RI 02903

     SEVENTH: The Corporation is to have perpetual existence.

     EIGHTH: Elections of Directors need not be by written ballot unless the
By-laws of the Corporation so provide.

     NINTH: In furtherance and not in limitation of the powers conferred by the
laws of Delaware, the Board of Directors of the Corporation is authorized and
empowered to adopt, alter, amend and repeal the By-laws of the Corporation in
any manner not inconsistent with the laws of Delaware.

     TENTH: The Corporation shall indemnify its officers, directors, employees
and agents to the greatest extent permitted by the General Corporation Law of
Delaware.

     ELEVENTH: Meetings of the stockholders may be held within or without the
State of Delaware, as the By-laws may provide. The books of the Corporation may
be kept (subject to any provision contained in the statutes) outside the State
of Delaware at such place or places as may be designated from time to time by
the Board of Directors or in the By-laws of the Corporation.

     TWELFTH: The Corporation reserves the right to amend, alter, change or
repeal any provision contained in this Certificate of Incorporation, in the
manner now or hereafter prescribed by statute.

     I, THE UNDERSIGNED, being the incorporator hereinbefore named, for the
purpose of forming a corporation pursuant to the General Corporation Law of
Delaware, do make this certificate, hereby declaring and certifying that this is
my act and deed and the facts herein stated are true, and accordingly have
hereunto set my hand this 18th day of November, 1999.


                                        ----------------------------------------
                                        Douglas G. Gray, Esq.
                                        Incorporator


                                       -2-