<PAGE>

                                                                   Exhibit 10.41


                             THERMISOL FINLAND OY

                              SECURITY AGREEMENT

                         Dated as of December 29, 1999

          In consideration of credit extended under Third Amended and Restated
Revolving Credit and Security Agreement dated as of December 29, 1999 ("the
Credit Agreement") between WINCUP HOLDINGS, INC., RADNOR CHEMICAL CORPORATION,
STYROCHEM U.S., LTD., RADNOR HOLDINGS CORPORATION, RADNOR DELAWARE, INC.,
STYROCHEM DELAWARE, INC. and WINCUP TEXAS, LTD. and STYROCHEM EUROPE (THE
NETHERLANDS) B.V., STYROCHEM FINLAND OY, THERMISOL DENMARK A/S, THERMISOL SWEDEN
AB and THERMISOL FINLAND OY, BANK OF AMERICA, N.A. and each of other financial
institutions which are and such financial institutions which become a party to
the "Credit Agreement" (Bank of America and such other financial institutions,
collectively, the "Lenders" and individually a "Lender") and Bank of America, as
administrative and collateral agent for the Lenders, the Agent and Thermisol
Finland Oy hereby agree as follows (with certain terms used herein being defined
in Article 1 of the Credit Agreement and in Article 6 of this Agreement):
<PAGE>

                                   ARTICLE 1

                               SECURITY INTEREST
                               -----------------

          Section 1.01 (a) Security Interest.  To secure the payment, observance
                           -----------------
and performance of the European Obligations, Thermisol Finland Oy hereby agrees
to establish a Business Mortgage (i.e. "yrityskiinnitys", also referred to in
English as a floating charge) in respect of the Collateral in favor of the Agent
for the ratable benefit of the Lenders. The Business Mortgage granted by
Thermisol Finland Oy is evidenced by Promissory Notes registered in the Finnish
Business Mortgage Register. The Promissory Notes are in the form set forth in
Exhibit 1. to this Agreement. By delivering such registered Promissory Notes to
the Agent or to the Agent's designated representative in Finland, the Borrowers
are entitled to draw under the Credit Agreement within the limits and under the
conditions set forth in the Credit Agreement. Thermisol Finland Oy will ensure
that the aggregate Dollar equivalent of the principal amount in Finnish Markka
of the registered Promissory Notes together with any other European Collateral
shall be sufficient to satisfy the requirement for the European Borrowing Base
set out in Section 2.1A in the Credit Agreement. Thermisol Finland Oy will,
forthwith upon demand from the Agent, deliver to the Agent or the Agent's
representative in Finland additional registered Promissory Note or Notes in an
amount which will ensure compliance with this Section 1.01(a).

          (b) Assigning,  Delivering and Pledging Existing Promissory Notes.
              --------------------------------------------------------------
Thermisol Finland Oy hereby agrees to the Prior U.K. Agent assigning and
delivering any and all of the current Promissory Notes registered in the Finnish
Business Mortgage Register and pledged in favor of the Prior U.K. Agent to the
Agent for the purpose of granting to the Agent the Business Mortgage described
in Section 1.01 (a) above.

          (c) Continued Priority of Security Interest.  The Business Mortgage
              ---------------------------------------
shall at all times be valid,  and enforceable against Thermisol Finland Oy and
all third parties, in accordance with the terms hereof, as security for the
European Obligations, and the Collateral shall not at any time be subject to any
Lien that is prior to, on a parity with or junior to the Busi-

                                       2
<PAGE>

ness Mortgage, except as required by mandatory Finnish law and as to any
Permitted Encumbrance.

          (d) Required Action. Thermisol Finland Oy shall take all action that
              ---------------
may be necessary or desirable, or that the Agent may request, so as at all times
to maintain the validity, enforceability and priority of the Business Mortgage
in the Collateral in conformity with the requirements of Section 1(c), or to
enable the Agent to protect, exercise or enforce the Lenders' rights hereunder
and in the Collateral, including but not limited to (i) executing the Promissory
Note(s) mentioned in Section 1.01 (a) and registering the same in the Finnish
Trade Register (ii) immediately discharging all Liens in respect of the
Collateral other than Permitted Encumbrances, and (iii) obtaining landlords' or
mortgagees' lien waivers (if any).

          Section 1.02 Thermisol Finland Oy  Remains Obligated; The Lenders Not
                       --------------------------------------------------------
Obligated.  The grant by Thermisol Finland Oy to the Lenders or to the Agent for
---------
the ratable benefit of the Lenders of the Business Mortgage  in the Collateral
shall not relieve Thermisol Finland Oy from the performance of any term,
covenant, condition or agreement on its part to be performed or observed under
or in respect of any of the Collateral or from any liability to a Person under
or in respect of any of the Collateral or impose any obligation on the Agent to
perform or observe any such term, covenant, condition or agreement on Thermisol
Finland Oy's part to be so performed or observed or impose any liability on the
Agent for any act or omission on the part of Thermisol Finland Oy relative
thereto.

          Section 1.03 Disposition of Collateral. Thermisol Finland Oy will make
                       -------------------------
no disposition of the Collateral except  in the ordinary course of business.

          Section 1.04 Defense of Agent's and Lenders' Interests. Until (a)
                       -----------------------------------------
payment and performance in full of all of the European Obligations and (b)
termination of the Credit Agreement as to the European Borrowers, the Business
Mortgage shall continue in full force and effect. During such period Thermisol
Finland Oy shall not, without the Agent's prior written consent, pledge, sell
(except Inventory and other Collateral in the ordinary course of business),
assign, transfer, create or suffer to exist a Lien upon or encumber or allow or
suffer to be encumbered in any way except for Permitted Encumbrances, any part
of the Collateral.

                                       3
<PAGE>

Thermisol Finland Oy shall defend Agent's and Lenders' interests in the
Collateral against any and all Persons whatsoever.

                                   ARTICLE 2

                          CERTAIN REPRESENTATIONS AND
                          ---------------------------
                      WARRANTIES OF THERMISOL FINLAND OY
                      ----------------------------------

     In order to induce the Lenders to extend credit under the Credit Agreement,
Thermisol Finland Oy represents and warrants as follows:

          Section 2.01 Organization; Power.  Thermisol Finland Oy is a
                       -------------------
corporation duly organized and validly existing under the laws of Finland, has
the corporate power and authority to own its property and to carry on its
business as now being conducted.

          Section 2.02 Authorization and Compliance of Agreement.  Styrochem
                       -----------------------------------------
Finland Oy has the corporate power, and has taken all necessary corporate action
to authorize it, to execute, deliver and perform this Agreement in accordance
with its terms. This Agreement has been duly executed and delivered by Thermisol
Finland Oy and is the legal, valid and binding obligation of Thermisol Finland
Oy. The execution, delivery and performance of this Agreement in accordance with
its terms does not and will not (i) require (a) any Governmental Approval (other
than the registration of the Business Mortgage by the Finnish National Board of
Patents and Registration), or (b) any consent or approval of the stockholders of
Thermisol Finland Oy, (ii) violate or conflict with, result in a breach of, or
constitute a default under, (a) any Contract to which Thermisol Finland Oy is a
party or by which any of its properties may be bound or (b) any applicable law
or (iii) result in or require the creation of any Lien upon any assets of
Thermisol Finland Oy, other than the Business Mortgage .

                                       4
<PAGE>

                                   ARTICLE 3

                                   COVENANTS
                                   ---------

          Unless the Lenders shall otherwise consent in writing:

          A. General.
             -------

          Section 3.01 Preservation of Existence, Scope of Business, Compliance
                       --------------------------------------------
with Law. Thermisol Finland Oy shall (a) preserve and maintain its corporate
existence, (b) engage only in businesses in substantially the same fields as the
businesses conducted on the day of signing this Agreement and (c) comply with
all applicable law.

          Section 3.02 (a) Ownership and Defense of Collateral. Thermisol
                           -----------------------------------
Finland Oy shall at all times (i) be the owner of the Collateral free from any
right, title or interest of any Person except Permitted Encumbrances and (ii)
defend the Collateral against the claims and demands of all Persons.

          (b) No Pledging of Securities. Thermisol Finland Oy shall not pledge
              -------------------------
any of its securities in favour of any other Person than the Lenders.

          Section 3.03 (a) Chief Executive Office. Thermisol Finland Oy shall
                           ----------------------
maintain its chief executive office and, if different from its chief executive
office, each office where the books and records relating to any Receivables are
kept only at, or, in the case of such books and records, in transit to, a
location of which the Agent has received not less than 60 days prior notice.

          (b) Change of Name, Identity, etc. Thermisol Finland Oy shall not
              -----------------------------
change its name or identity without the Agent's permission in writing.

          (c) Invoices.  Thermisol Finland Oy shall not use any invoice
              --------
identifying the seller of the goods or provider of the services to which such
invoice relates by a name other than Thermisol Finland Oy's corporate name
provided, however, that Thermisol Finland Oy may use any parallel or
supplementary trade name, provided it has been registered in the Trade Register.

          Section 3.04 Taxes; Compliance.  Thermisol Finland Oy shall (a) pay or
                       -----------------
cause to be paid all taxes, assessments and governmental charges levied or
assessed or imposed

                                       5
<PAGE>

upon or with respect to any of the Collateral and (b) comply with (i) all laws,
rules and regulations applicable to its business or the Collateral and (ii) the
terms and provisions of all leases, mortgages, and other agreements relating to
premises whereby Collateral is located.

          Section 3.05 (a) Property Insurance. (i) Thermisol Finland Oy shall
                           ------------------
insure the Inventory against loss or damage by fire, theft, burglary, pilferage,
loss in transit and such other hazards as the Agent shall from time to time
specify, in amounts and under policies issued by insurers acceptable to the
Agent, and all premiums thereon shall be paid by Thermisol Finland Oy and copies
of the policies delivered to the Agent.(ii) All insurance policies required
under this Section 3.05(a) shall contain loss payable clauses in the form and
substance satisfactory to the Agent, and shall name the Agent for the ratable
benefit of the Lenders as loss payee as its interests may appear, and provide
(A) that all proceeds thereunder in excess of USD 100,000 or the equivalent
amount in Finnish Markka, the lawful currency of Finland, shall be payable to
the Agent for the ratable benefit of the Lenders, (B) no such insurance shall be
affected by any act or neglect of the insurer or owner of the property described
in such policy, and (C) that such policy and loss payable clauses may not be
canceled, amended or terminated unless at least 30 days prior notice is given to
the Agent, (iii) Proceeds of such insurance paid to the Lenders or to the Agent
for the ratable benefit of the Lenders shall, prior to an Event of Default, be
held by the Agent as cash collateral and, after an Event of Default, at the
option of the Agent, be so held or applied to the payments of such of the
Obligations as the Agent may elect to the extent permitted by law. (iv)
Thermisol Finland Oy will not use or permit any part of the Inventory or any
part of the Equipment to be used unlawfully or outside of any insurance
coverage.

          Section 3.06 Inspection.  The Lenders (by any of its officers or
                       ----------
employees) shall have upon reasonable prior notice the right at any time or
times during the business hours of Thermisol Finland Oy to (a) inspect the
Collateral, all records and files relating thereto and the premises upon which
any of the Collateral is located, (b) discuss with any Person, and each such
Person is hereby authorized to discuss with the Lenders, Styrochem  Finland Oy's
business, assets, liabilities, financial condition, results of operation and
business prospects, provided that the Lenders inform Thermisol Finland Oy of
such discussions prior

                                       6
<PAGE>

to or as soon as possible after they have occurred, (c) upon reasonable prior
notice verify the amount, quantity, value and condition of, or any other matter
relating to, any of the Collateral and (d) upon reasonable prior notice review,
audit and make extracts from all records and files related to any of the
Collateral. Thermisol Finland Oy hereby authorizes each of the Lenders and the
Agent, and will deliver to the Lenders and the Agent any instrument necessary
for them, to obtain records from any service bureau maintaining records on
behalf of Thermisol Finland Oy.

          Section 3.07 Maintenance.  Thermisol Finland Oy shall maintain all
                       -----------
physical property that constitutes Collateral in good condition, with reasonable
allowance for wear and tear, and shall exercise proper custody over all such
property.

          Section 3.08 Notice of Diminution of Value.  Thermisol Finland Oy
                       -----------------------------
shall give prompt notice to the Agent of any matter or event which has resulted
in, or may result in, the actual or potential diminution in value of any of its
Collateral other than diminutions which are insignificant in amount.

          Section 3.09 Information.  In addition to such other information as
                       -----------
shall be specifically provided for herein, Thermisol Finland Oy shall furnish to
the Agent such other information with respect to the Collateral as the Agent may
reasonably request from time to time.

          B. Receivables.
             -----------

          Section 3.10 (a) Status of Receivables.  Each Receivable (i) shall at
                           ---------------------
all times represent the legal, valid and binding obligation of the Customer and
at the time it arose be enforceable in accordance with its terms, (ii) shall at
no time be subject to (A) any defense, set-off or counter-claim, other than ones
arising in the ordinary course of business for refunds or allowances resulting
from damaged or non-complying merchandise, (B) any agreement prohibiting
assignment or requiring notice of or consent to assignment, or (iii) shall arise
out of a bonafide transaction, (iv) represented by any outstanding invoice shall
require no further act on the part of Thermisol Finland Oy or any other Person
to make it payable by the Customer, and (v) shall comply with all applicable
laws.

                                       7
<PAGE>

          (b) Patents, Etc. Thermisol Finland Oy  shall maintain, and diligently
              ------------
pursue all applications for, all patents, copyrights, trademarks, service marks,
trade names and the like belonging to its business.

          Section 3.11 No Sales of Receivables.  Thermisol Finland Oy shall not
                       -----------------------
sell any Receivables or any interest therein, or pledge any Receivable in favour
of any other Person than the Lenders, except as permitted under Section 1.04
above.

          Section 3.12 Collection. Thermisol Finland Oy shall endeavor to cause
                       ----------
to be collected from the Customer of each Receivable, as and when due
(including, without limitation, Receivables which are delinquent, such
Receivables to be collected in accordance with lawful collection procedures) any
and all amounts owing under or on account of such Receivable, and apply
forthwith upon receipt thereof all such amounts as are so collected to the
outstanding balance of such Receivable.

          C.  Inventory.
              ---------

          Section 3.13 Nature of Inventory.  All Inventory shall at all times
                       -------------------
(i) be in good condition, (ii) meet all governmental standards applicable
thereto or to its use or sale, and (ii) be currently either usable or salable in
the ordinary course of  Thermisol Finland Oy's business.

          Section 3.14 Records. Thermisol Finland Oy shall keep correct and
                       -------
accurate records of Inventory, itemizing and describing the kind, type, and
quantity of Inventory, Thermisol Finland Oy's cost therefor and a current price
list for such Inventory.

          Section 3.15 (a) No Sales of Inventory. Thermisol Finland Oy shall
                           ---------------------
without the prior written consent by the Agent not sell any Inventory or any
interest therein except for sales in the ordinary course of its business.

          (b) Sales in Compliance With Applicable Law. All sales of Inventory
              ---------------------------------------
will be made in full compliance with all requirements of applicable law.

                                       8
<PAGE>

                                   ARTICLE 4

                REMEDIES UPON OCCURRENCE OF AN EVENT OF DEFAULT
                -----------------------------------------------

          If any Event of Default shall have occurred and be continuing, then
and in such case:

          Section 4.01 (a) No Notice or Compensation. Each of the Lenders and
                           -------------------------
the Agent may exercise any or all of its rights and remedies hereunder and under
applicable laws and regulations without (A) notice to or demand upon Thermisol
Finland Oy, (B) resistance or interference by Thermisol Finland Oy and (C)
payment of compensation of any kind to Thermisol Finland Oy.

          Section 4.02 Application of Proceeds. The Agent on behalf of itself
                       -----------------------
and all of the Lenders hereby commit themselves and their successors,
transferees and assigns not to collect their claims under the Security Agreement
from proceeds received from the realization of any fixed assets of Thermisol
Finland Oy or other assets not constituting part of the Collateral. If at any
time any of the Lenders or any of their successors, transferees or assigns
receives any proceeds from the realization of any fixed assets of Thermisol
Finland Oy or other assets not constituting part of the Collateral, such
receiving party will immediately pay the full amount of such proceeds received
to Thermisol Finland Oy.

                                   ARTICLE 5

                                 MISCELLANEOUS

          Section 5.01 Registration Charges. Thermisol Finland Oy will pay any
                       --------------------
and all registration and similar taxes, fees or charges, and shall indemnify
each of the Lenders and the Agent against any and all liabilities with respect
to or resulting from any delay in the payment or omission to pay any such taxes,
fees or charges, which may be payable or determined to be payable in connection
with the execution, delivery, performance or enforcement of this Agreement or
any instrument, document or the like executed and delivered (i) in connection
with, or contemplated by, this Agreement, or (ii) in con-

                                       9
<PAGE>

nection with any transaction in connection with or contemplated by this
Agreement.

          Section 5.02 (a) Expenses of Thermisol Finland Oy's Agreements and
                           -------------------------------------------------
Duties.  Thermisol Finland Oy's agreements and duties hereunder and under the
------
other Collateral Documents shall be performed by it at its sole cost and
expense.

          (b) Lenders' Right to Perform on Thermisol Finland Oy's Behalf. If
              ----------------------------------------------------------
Styro-chem Finland Oy shall fail to do any act or thing which it has agreed to
do hereunder or under the other Collateral Documents, or fails to observe or
perform any agreement or obligation hereunder, or under the other Collateral
Documents, the Agent may (but shall not be obligated to) do the same or cause it
to be done or performed or observed, either in its name or in the name or on
behalf of Thermisol Finland Oy, and Thermisol Finland Oy hereby irrevocably
authorizes the Agent so to do.

          (c) Expenses; Indemnification. Thermisol Finland Oy shall pay, or
              -------------------------
reimburse the Agent for the ratable benefit of the Lenders for, all of the
Lenders' reasonable costs and expenses (including fees and disbursements of
legal counsel, appraisers, accountants, and other experts, employed or retained
by the Lenders or the Agent) incurred, and indemnify and hold the Lenders and
the Agent harmless from and against all losses suffered by the Lenders and the
Agent in connection with, (i) the negotiation, preparation, execution and
delivery of (A) this Agreement or any of the other Collateral Documents and (B)
whether or not executed, any waiver, amendment or consent thereunder, (ii)
consulting with respect to any matter in any way arising out of, related to, or
connected with, this Agreement or any of the other Collateral Documents,
including but not limited to the enforcement of any of the Obligations or the
enforcement or defense by the Lenders and the Agent of the Security Interest as
a valid first priority Business Mortgage , (iii) protecting, preserving,
exercising or enforcing the rights of the Lenders under this Agreement or any of
the other Collateral Documents, and (iv) any claim (whether asserted by the
Lenders, Thermisol Finland Oy or any other entity), and the prosecution or
defense thereof, in any way arising under, out of, related to, or connected
with, this Agreement or any of the other Collateral Documents. Thermisol Finland
Oy shall also pay (i) to the Agent for the ratable benefit of the Lenders its
charges for all

                                       10
<PAGE>

bank services performed by it for Thermisol Finland Oy at Thermisol Finland Oy's
request and (ii) all transfer, documentary, stamp and similar taxes, and all
recording and filing fees, payable in connection with, arising out of, or in any
way connected with, the execution, delivery or performance of this Agreement or
any of the other Collateral Documents.

          Section 5.03 No Waivers of Rights Hereunder; Amendments.  No course of
                       ------------------------------------------
dealing or performance by the Agent or the Lenders, including any delay or
forbearance by them in exercising any right hereunder or under any of the
Collateral Documents, shall operate as a waiver by them of such right, or the
amendment, release or novation of any provision hereof or thereof, nor shall any
single or partial exercise of any right preclude other or further exercises
thereof or the exercise of any other right.  No waiver of any of their rights
under, nor any amendment of any provision of, this Agreement or any of the
Collateral Documents, shall be enforceable against the Lenders unless in writing
and signed by an authorized officer of the Lenders, and unless it expressly
refers to the right or provision affected; any such waiver shall be limited
solely to the specific event waived.

          Section 5.04 Assignment. The Lenders may assign any or all of the
                       ----------
Obligations and may transfer therewith any or all of the Collateral therefor
and the transferee shall have the same rights with respect thereto as had the
Lenders. Upon such transfer, the Lenders shall be released from all
responsibility for the Collateral so transferred.

          Section 5.05 (a) Governing Law. This Agreement shall be governed by
                           -------------
and interpreted and enforced in accordance with the laws of Finland, and the
Lenders shall have the rights and remedies of a secured party under such laws.

          (b) Jurisdiction. All actions and proceedings in any way arising out
              ------------
of, related to, or connected with, this Agreement and any of the Collateral
Documents shall be litigated in the District Court of Helsinki (Helsingin
karajaoikeus) in Finland, and, in the case of any such commenced by the Lenders,
in such other courts as the Lenders may select. Such courts are convenient
forums for Thermisol Finland Oy and Thermisol Finland Oy submits to the personal
jurisdiction of such courts.

                                       11
<PAGE>

          (c) Certain Waivers.  Thermisol Finland Oy waives the right to assert
              ---------------
in any action or proceeding between  it and the Lenders any offsets or
counterclaims which it may have.

          (d) Manner of Payment and Performance.  All payments due to the
              ---------------------------------
Lenders  hereunder, and all other terms, conditions, covenants and agreements to
be observed or performed by Thermisol Finland Oy hereunder, shall be made,
observed or performed by Thermisol Finland Oy without any reduction whatsoever,
including but not limited to any reduction for any setoff, recoupment,
counterclaim or tax.

          Section 5.06 Notices. Any notice or request hereunder may be given to
                       -------
Styro-chem Finland Oy or to the Agent as provided for in the Credit Agreement,
Section 15.6. Notice.

          Section 5.07 (a) This Agreement and the Collateral Documents shall be
binding upon the heirs, executors, administrators, assigns or successors of
Thermisol Finland Oy and shall inure to the benefit of and be enforceable by the
Lenders, its successors, transferees and assigns.

          (b) Any provision of this Agreement which is prohibited or
unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective
to the extent of such prohibition or unenforceability without invalidating the
remaining provisions hereof in that jurisdiction or affecting the validity or
enforceability of such provision in any other jurisdiction.

                                   ARTICLE 6

                                  DEFINITIONS
                                  -----------

          Section 6.01 (a) Certain Terms Defined by Reference. (i) All terms
                           ----------------------------------
defined in the Credit Agreement are used herein with the meanings therein given.

          (b) Other Defined Terms. For purpose of this Agreement:
              --------------------

"Agreement" means this Agreement.

                                       12
<PAGE>

"Business Mortgage" (translation of the Finnish word "Yrityskiinnitys") means a
mortgage in certain property, as defined in Article 3 of the Act on Business
Mortgages 24.8.1984/634 (in Finnish "Yrityskiinnityslaki") with amendments, of a
Finnish business enterprise registered in the Finnish Trade Register.

"Collateral" means and includes all of Thermisol Finland Oy's Inventory,
Receivables, General Intangibles (excluding tax refunds) as defined in the
Credit Agreement and which Collateral is covered by a Business Mortgage pursuant
to the Finnish Act on Business Mortgages 24.8.1984/634 with amendments.

"Collateral Documents" means agreements, documents and instruments related to,
arising out of, or in any way connected with (a) this Agreement including but
not limited to (i) each Promissory Note ("Panttivelkakirja") to be registered in
the Finnish Business Mortgage Register and (ii) thereto related application form
("Yrityskiinnityshakemus") and, (b) any other agreement, document or instrument
referred to in this clause (b), in each case whether now or hereafter executed.

"Person" means an individual, corporation, partnership, trust or unincorporated
organization or a government or any agency or subdivision thereof.

"Promissory Note" means a Note granting either the holder of the note or the
named creditor of the note, as the case may be, in accordance with Article 2 of
the Act on Business Mortgages 24.8.1984/634 with amendments evidencing the
consent of Thermisol Finland Oy to grant a registration of a claim for the
capital, interest and collecting costs, in amounts which are determined in each
Promissory Note.

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

THERMISOL FINLAND OY                     BANK OF AMERICA

By:/s/ R. Radcliffe Hastings             By:/s/ Donald J. Chin
   ____________________________             --------------------------
Name:R. Radcliffe Hastings               Name:Donald J Chin
Title: Authorized Representative         Title:Managing Director

                                       13