Exhibit 3.21


                                                               STATE OF DELAWARE
                                                              SECRETARY OF STATE
                                                        DIVISION OF CORPORATIONS
                                                       FILED 04:00 PM 06/06/1991
                                                            751157057 -- 2138616




                            CERTIFICATE OF AMENDMENT

                                     OF THE

                          CERTIFICATE OF INCORPORATION

                                       OF

                             GAF EXPORT CORPORATION

                        Pursuant to Sections 228 and 242
                         of the General Corporation Law
                            of the State of Delaware

            Stephen A. Block, Senior Vice President of GAF Export Corporation
(the "Corporation"), does hereby certify as follows:

            1. The Certificate of Incorporation of the Corporation was filed in
the Office of the Secretary of State of the State of Delaware on the 23rd day of
September 1987 under the name Newco Mirror 2 Corp.

            2. The Corporation elects, pursuant to Section 242 of the General
Corporation Law of the State of Delaware, to amend article "FIRST" of the
Certificate of Incorporation to read in its entirety as follows:

                  "FIRST: The name of the Corporation is ISP (Puerto Rico) Inc."

            3. Such amendment has been duly adopted in accordance with the
provisions of Section 242 of the General Corporation Law of the State of
Delaware, by the written consent, in accordance with the provisions of Section
228 of such statute, of the holder of all outstanding stock entitled to vote
thereon.

            IN WITNESS WHEREOF, the Corporation has duly caused this Certificate
of Amendment to be executed on its behalf by its Senior Vice President and
attested by its Assistant Secretary, this 28th day of May, 1991.


                                      GAF EXPORT CORPORATION


                                      By: /s/ Stephen A. Block
                                         --------------------------------------
                                         Stephen A. Block
                                         Senior Vice President

Attest:


/s/ Deborah D. Lawson
-----------------------------
Deborah D. Lawson
Assistant Secretary







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                            CERTIFICATE OF AMENDMENT

                                     TO THE

                          CERTIFICATE OF INCORPORATION

                                       OF

                                   CLOVER INC.


                         ADOPTED IN ACCORDANCE WITH THE
                        PROVISIONS OF SECTION 242 OF THE
                        DELAWARE GENERAL CORPORATION LAW

            It is hereby certified that:

            1. The present name of the corporation (the "Corporation") is Clover
Inc.

            2. The Certificate of Incorporation of the Corporation was filed
with the Secretary of State of Delaware on September 23, 1987.

            3. Article FIRST of the Certificate of Incorporation of the
Corporation is hereby amended to read in its entirety as follows:

            FIRST: The name of the corporation (hereinafter called the
            "corporation") is GAF Export Corporation.

            4. The foregoing amendment was declared advisable by the sole
director of the Corporation pursuant to a resolution duly adopted the amendment
on March 27, 1989, and was duly adopted in accordance with the provisions of
Section 242 of the Delaware General Corporation Law by the affirmative vote of
the sole stockholder of the Corporation.

            IN WITNESS WHEREOF, the Corporation has caused this Certificate to
be signed on this 31 day of March, 1989.


                                      CLOVER INC.


                                      By: /s/ Samuel J. Heyman
                                         --------------------------------------
                                         Samuel J. Heyman
                                         President and Sole Director

Attest:


/s/ Irving Kagan
------------------------------
Irving Kagan
Vice President and Secretary







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                            CERTIFICATE OF AMENDMENT

                                       OF

                          CERTIFICATE OF INCORPORATION

                                       OF

                              NEWCO MIRROR 2 CORP.

                         ADOPTED IN ACCORDANCE WITH THE
                        PROVISIONS OF SECTION 241 OF THE
                         GENERAL CORPORATION LAW OF THE
                                STATE OF DELAWARE

            NEWCO MIRROR 2 CORP., a corporation organized and existing under and
by virtue of the General Corporation Law of the State of Delaware (the
"Corporation"), hereby sets forth an Amendment of its Certificate of
Incorporation pursuant to Section 241 of the General Corporation Law of the
State of Delaware (the "DGCL"), hereby certifying as follows:

            FIRST: Article FIRST of the Certificate of Incorporation of the
Corporation is hereby amended in its entirety to read as follows:

            "FIRST: The name of the corporation (the "corporation") is Clover
            Inc."

            SECOND: The Corporation has not received any payment for any of its
stock.

            THIRD: The foregoing amendment has been duly adopted in accordance
with the provisions of Section 241 of the DGCL.

            IN WITNESS WHEREOF, the Corporation has caused this Certificate to
be signed by Samuel J. Heyman, its President, and Irving Kagan, its Secretary,
this 24th day of February, 1988.


                                    NEWCO MIRROR 2 CORP.

                                    By: /s/ Samuel J. Heyman
                                       --------------------------------------
                                       Samuel J. Heyman
                                       President

Attest:


/s/ Irving Kagan
-------------------------------
Irving Kagan
Secretary








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                          CERTIFICATE OF INCORPORATION

                                       OF

                              NEWCO MIRROR 2 CORP.

                                  ------------

            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

                  NEWCO MIRROR 2 CORP.

            SECOND: The address, including street, number, city, and county, of
the registered office of the corporation in the State of Delaware is 229 South
State Street, City of Dover, County of Kent; and the name of the registered
agent of the corporation in the State of Delaware at such address is The
Prentice-Hall Corporation System, Inc.

            THIRD: The purpose of the corporation is 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 One Thousand (1,000). The par value of each of
such shares is One Cent ($.0l). All such shares are of one class and are shares
of Common Stock.

            FIFTH: The name and the mailing address of the incorporator are as
follows:



                NAME                     MAILING ADDRESS
                ----                     ---------------
                       
             N.S. Truax   229 South State Street, Dover, Delaware 19901


            SIXTH: The corporation is to have perpetual existence.

            SEVENTH: Whenever a compromise or arrangement is proposed between
this corporation and its creditors or any class of them and/or between this
corporation and its stockholders or any class of them, any court of equitable
jurisdiction

within the State of Delaware may, on the application in a summary way of this
corporation or of any creditor or stockholder thereof or on the application of
any receiver or receivers appointed for this corporation under the provisions of
section 291 of Title 8 of the Delaware Code or on the application of trustees in
dissolution or of any receiver or receivers appointed for this corporation under
the provisions of section 279 of Title 8 of the Delaware Code order a meeting of
the creditors or class of creditors, and/or of the stockholders or class of
stockholders of this corporation, as the case may be, to be summoned in such
manner as the said court directs. If a majority in number representing
three-fourths in value of the creditors or class of creditors, and/or of the
stockholders or class of stockholders of this corporation, as the case may be,
agree to any compromise or arrangement and to any reorganization of this
corporation as consequence of such compromise or arrangement, the said
compromise or arrangement and the said reorganization shall, if sanctioned by
the court to which the said application has been made, be binding on all the
creditors or class of creditors, and/or on all the stockholders or class of
stockholders, of this corporation, as the case may be, and also on this
corporation.

            EIGHTH: For the management of the business and for the conduct of
the affairs of the corporation, and in further definition, limitation and
regulation of the powers of the corporation and of its directors and of its
stockholders or any class thereof, as the case may be, it is further provided:

            1. The management of the business and the conduct of the affairs of
      the corporation shall be vested in its Board of Directors. The number of
      directors which shall constitute the whole Board of Directors shall be
      fixed by, or in the manner provided in, the By-Laws. The phrase "whole
      Board" and the phrase "total number of directors" shall be deemed to have
      the same meaning, to wit, the total number of directors which the
      corporation would have if there were no vacancies. No election of
      directors need be by written ballot.

            2. After the original or other By-Laws of the corporation have been
      adopted, amended, or repealed, as the case may be in accordance with the
      provisions of Section 109 of the General Corporation Law of the State of
      Delaware, and, after the corporation has received any payment for any of
      its stock, the power to adopt, amend, or repeal the By-Laws of the
      corporation may be exercised by the Board of Directors of the corporation;
      provided, however, that any provision for the classification of directors
      of the corporation for staggered terms pursuant to the provisions of
      subsection (d) of Section 141 of the General Corporation Law of the State
      of Delaware shall be set forth in an initial By-Law or in a By-Law adopted
      by the stockholders entitled to vote of the corporation unless provisions
      for such classification shall be set forth in this certificate of
      incorporation.



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            3. Whenever the corporation shall be authorized to issue only one
      class of stock, each outstanding share shall entitle the holder thereof to
      notice of, and the right to vote at, any meeting of stockholders. Whenever
      the corporation shall be authorized to issue more than one class of stock,
      no outstanding share of any class of stock which is denied voting power
      under the provisions of the certificate of incorporation shall entitle the
      holder thereof to the right to vote at any meeting of stockholders except
      as the provisions of paragraph (2) of subsection (b) of section 242 of the
      General Corporation Law of the State of Delaware shall otherwise require;
      provided, that no share of any such class which is otherwise denied voting
      power shall entitle the holder thereof to vote upon the increase or
      decrease in the number of authorized shares of said class.

            NINTH: The personal liability of the directors of the corporation is
hereby eliminated to the fullest extent permitted by paragraph (7) of subsection
(b) of Section 102 of the General Corporation Law of the State of Delaware, as
the same may be amended and supplemented.

            TENTH: The corporation shall, to the fullest extent permitted by
Section 145 of the General Corporation Law of the State of Delaware, as the same
may be amended and supplemented, indemnify any and all persons whom it shall
have power to indemnify under said section from and against any and all of the
expenses, liabilities or other matters referred to in or covered by said
section, and the indemnification provided for herein shall not be deemed
exclusive of any other rights to which those indemnified may be entitled under
any By-Law, agreement, vote of stockholders or disinterested directors or
otherwise, both as to action in his official capacity and as to action in
another capacity while holding such office, and shall continue as to a person
who has ceased to be a director, officer, employee or agent and shall inure to
the benefit of the heirs, executors and administrators of such a person.

            ELEVENTH: From time to time any of the provisions of this
certificate of incorporation may be amended, altered or repealed, and other
provisions authorized by the laws of the State of Delaware at the time in force
may be added or inserted in the manner and at the time prescribed by said laws,
and all rights at any time conferred upon the stockholders of the corporation by
this certificate of incorporation are granted subject to the provisions of this
Article ELEVENTH.

Signed on September 23, 1987


                                     /s/ N.S. Truax
                                    ------------------------------------
                                                 N.S. Truax
                                                Incorporator




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