<PAGE>   1
                                                                   Exhibit 10.21

                                    SUPPLEMENT NO. 1 dated as of July 19, 2001,
                                    to the Indemnity, Subrogation and
                                    Contribution Agreement dated as of September
                                    30, 1997, as amended and restated as of May
                                    31, 2000 (as the same may be amended,
                                    supplemented or otherwise modified from time
                                    to time, the "Indemnity, Subrogation and
                                    Contribution Agreement"), among PLIANT
                                    CORPORATION (f/k/a Huntsman Packaging
                                    Corporation), a Utah corporation (the
                                    "Borrower"), each Subsidiary of the Borrower
                                    listed on Schedule I thereto (the
                                    "Guarantors"), and BANKERS TRUST COMPANY, a
                                    New York banking corporation ("Bankers
                                    Trust"), as collateral agent (the
                                    "Collateral Agent") for the Secured Parties
                                    (as defined in the Credit Agreement referred
                                    to below).


         A. Reference is made to (a) the Credit Agreement dated as of September
30, 1997, as amended and restated as of May 31, 2000 (as amended, supplemented
or otherwise modified from time to time, the "Credit Agreement"), among the
Borrower, Aspen Industrial, S.A. de C.V., a Mexico corporation, the lenders from
time to time party thereto (the "Lenders") and Bankers Trust, as administrative
agent (in such capacity, the "Administrative Agent") for the Lenders, and (b)
the Guarantee Agreement dated as of September 30, 1997, as amended and restated
as of May 31, 2000 (the "Guarantee Agreement"), among the Borrower, the
Guarantors and the Administrative Agent.

         B. Capitalized terms used herein and not otherwise defined herein shall
have the meanings assigned to such terms in the Indemnity, Subrogation and
Contribution Agreement and the Credit Agreement.

         C. The Borrower and the Guarantors have entered into the Indemnity,
Subrogation and Contribution Agreement in order to induce the Lenders to make
Loans and the Issuing Bank to issue Letters of Credit. Pursuant to Section 5.12
of the Credit Agreement, each Subsidiary Loan Party that was not in existence or
not a Subsidiary Loan Party on the date of the Credit Agreement is required to
enter into the Guarantee Agreement as a Guarantor upon becoming a Subsidiary.
Section 12 of the Indemnity, Subrogation and Contribution Agreement provides
that additional Subsidiaries of the Borrower may become Guarantors under the
Indemnity, Subrogation and Contribution Agreement by execution and delivery of
an instrument in the form of this Supplement. Each of the undersigned
Subsidiaries of the Borrower (the "New Guarantors") is executing this Supplement
in accordance with the requirements of the Credit Agreement to become a
Guarantor under the Indemnity, Subrogation and Contribution Agreement in order
to induce the Lenders to make additional Loans and the Issuing Bank to issue
additional Letters of Credit and as consideration for Loans previously made and
Letters of Credit previously issued.

Accordingly, the Collateral Agent and each of the New Guarantors agree as
follows:
<PAGE>   2
         SECTION 1. In accordance with Section 12 of the Indemnity, Subrogation
and Contribution Agreement, each of the New Guarantors by its signature below
becomes a Guarantor under the Indemnity, Subrogation and Contribution Agreement
with the same force and effect as if originally named therein as a Guarantor and
each of the New Guarantors hereby agrees to all the terms and provisions of the
Indemnity, Subrogation and Contribution Agreement applicable to it as a
Guarantor thereunder. Each reference to a "Guarantor" in the Indemnity,
Subrogation and Contribution Agreement shall be deemed to include each of the
New Guarantors. The Indemnity, Subrogation and Contribution Agreement is hereby
incorporated herein by reference.

         SECTION 2. Each of the New Guarantors represents and warrants to the
Collateral Agent and the other Secured Parties that this Supplement has been
duly authorized, executed and delivered by it and constitutes its legal, valid
and binding obligation, enforceable against it in accordance with its terms.

         SECTION 3. This Supplement may be executed in counterparts (and by
different parties hereto on different counterparts), each of which shall
constitute an original, but all of which when taken together shall constitute a
single contract. This Supplement shall become effective when the Collateral
Agent shall have received counterparts of this Supplement that, when taken
together, bear the signatures of each of the New Guarantors and the Collateral
Agent. Delivery of an executed signature page to this Supplement by facsimile
transmission shall be as effective as delivery of a manually signed counterpart
of this Supplement.

         SECTION 4. Except as expressly supplemented hereby, the Indemnity,
Subrogation and Contribution Agreement shall remain in full force and effect.

         SECTION 5. THIS SUPPLEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

         SECTION 6. In case any one or more of the provisions contained in this
Supplement should be held invalid, illegal or unenforceable in any respect, no
party hereto shall be required to comply with such provision for so long as such
provision is held to be invalid, illegal or unenforceable, but the validity,
legality and enforceability of the remaining provisions contained herein and in
the Indemnity, Subrogation and Contribution Agreement shall not in any way be
affected or impaired. The parties hereto shall endeavor in good-faith
negotiations to replace the invalid, illegal or unenforceable provisions with
valid provisions the economic effect of which comes as close as possible to that
of the invalid, illegal or unenforceable provisions.

         SECTION 7. All communications and notices hereunder shall be in writing
and given as provided in Section 7 of the Indemnity, Subrogation and
Contribution Agreement. All communications and notices hereunder to each of the
New Guarantor shall be given to it at the address set forth under its signature.
<PAGE>   3
         SECTION 8. Each of the New Guarantors agrees to reimburse the
Collateral Agent for its reasonable out-of-pocket expenses in connection with
this Supplement, including the reasonable fees, other charges and disbursements
of counsel for the Collateral Agent.
<PAGE>   4
IN WITNESS WHEREOF, each of the New Guarantors and the Collateral Agent have
duly executed this Supplement to the Indemnity, Subrogation and Contribution
Agreement as of the day and year first above written.


                                        UNIPLAST HOLDINGS INC.,

                                        By    /s/ Larry Shepler
                                            ------------------------------------
                                        Name:     Larry Shepler
                                        Title:    Secretary
                                        Address:  c/o Pliant Corporation
                                                  1515 Woodfield Road, Suite 600
                                                  Schaumburg, Illinois 60173


                                        UNIPLAST U.S., INC.,

                                        By    /s/ Larry Shepler
                                            ------------------------------------
                                        Name:     Larry Shepler
                                        Title:    Secretary
                                        Address:  c/o Pliant Corporation
                                                  1515 Woodfield Road, Suite 600
                                                  Schaumburg, Illinois 60173


                                        TUREX, INC.,

                                        By    /s/ Larry Shepler
                                            ------------------------------------
                                        Name:     Larry Shepler
                                        Title:    Secretary
                                        Address:  c/o Pliant Corporation
                                                  1515 Woodfield Road, Suite 600
                                                  Schaumburg, Illinois 60173


                                        PIERSON INDUSTRIES, INC.,

                                        By    /s/ Larry Shepler
                                            ------------------------------------
                                        Name:     Larry Shepler
                                        Title:    Secretary
                                        Address:  c/o Pliant Corporation
                                                  1515 Woodfield Road, Suite 600
                                                  Schaumburg, Illinois 60173
<PAGE>   5
                                        UNIPLAST MIDWEST, INC.,

                                        By    /s/ Larry Shepler
                                            ------------------------------------
                                        Name:     Larry Shepler
                                        Title:    Secretary
                                        Address:  c/o Pliant Corporation
                                                  1515 Woodfield Road, Suite 600
                                                  Schaumburg, Illinois 60173


                                        BANKERS TRUST COMPANY,
                                        as Collateral Agent,

                                        By   /s/ Robert R. Telesca
                                           -------------------------------------
                                        Name: Robert R. Telesca
                                        Title: Vice President
                                        Address: One Bankers Trust Plaza
                                                 New York, New York 10006