Formulation of a Polyester Putty

SpecialChem | Edward M. Petrie - Jan 16, 2008


For many of us, the first adhesive / sealant that we used was an auto body patching compound. Relatively simple to use, inexpensive and functional, this all-purpose sealing, smoothing, and mortar material is still a mainstay in many repair shops and garages. Today's editorial describes its formulation.

Polyesters are a large class of synthetic resins having widely varying properties. Polyester resin based adhesives are relatively low cost formulations that have found niche applications. They may be divided into two distinctive groups: saturated (thermoplastic) and unsaturated (thermosetting).

Unsaturated polyester resins are commonly used for casting, glass fiber laminates, and adhesive systems. Unsaturated polyesters are fast-curing, two-part systems that harden by the addition of catalysts, usually peroxides. Styrene monomer is generally used as a reactive diluent for polyester resins. Cure can occur at room or elevated temperatures depending on the type of catalyst. Accelerators, such as cobalt naphthalene, are sometimes incorporated into the resin to speed cure.

Unsaturated polyester adhesives exhibit greater shrinkage during cure and poorer chemical resistance than epoxy adhesives. Certain types of polyesters are inhibited from curing by the presence of air leaving a tacky resin surface. However, they cure fully when fully protected from air by covering or enclosing between two substrates. Depending on the type of polyester resin used, the adhesives can be quite flexible or very rigid. Tensile shear strengths from several hundred to thousands of psi can be realized. Polyester adhesives bond to metals, ceramics, and glass reinforced laminates. Applications include patching kits for the repair of automobile bodies and repair cement for concrete flooring. Polyester adhesives also have strong bond strength to glass-reinforced polyester laminates.

For a body patching compound to be acceptable in automobile and other transportation applications it must have:

  1. Cohesive strength: to resist thermal and mechanical stress cracking.
  2. Sandability: to permit abrasive smoothing without gumming, cracking, or tearing prior to painting.
  3. Paint compatibility: Adhesion, appearance, environmental resistance and non-bleeding characteristics for the life of the vehicle.
  4. Processability: easy application to surfaces that are less than optimally clean; the compound must not be too runny (yet be capable of smoothing) and set up rapidly but not too rapidly.

Epoxy, urethane, and polyesters have mainly been used in these applications. Their properties are compared in Table 1. Unsaturated polyester resins are perhaps the most commonly used in formulations for automotive body putty. A starting formulation is provided in Table 2. This formulation will cure moderately quickly at room temperature.

Epoxy
Urethane
Polyester
Adhesion
High
High
High
Cohesive strength
High
Medium
Medium
Paint compatibility
High
Medium
High
Resistance to:
  • Moisture
  • Solvents

  • High
    High

    Variable*
    Variable*

    Medium
    Medium
    Chip resistance
    Medium
    High
    Medium
    Sandability
    High
    Medium
    High
    Processability
    Acceptable
    Difficult
    Excellent
    Cost
    High
    High
    Low
    Table 1: General Properties of Body Patching Resins

    Components
    Parts by Weight
    Part A
    Polyester resin (with styrene monomer)
    100
    Cobalt napththenate
    0.1
    Calcium carbonate
    As required for viscosity
    Part B
    Methyl ethyl ketone peroxide
    1.0
    Table 2: Starting Formulation for a Polyester Body Patching Compound

    Methyl ethyl ketone peroxide (MEKP) is generally used as the catalyst in these formulations along with an accelerator such as cobalt naphthalene. They will cause complete crosslinking of the polyester in a relatively short time at low temperatures. The working time and cure time will depend on the temperature and on the concentrations of the catalyst and accelerator in the formulation. Fillers such as calcium carbonate and silica are generally used to provide a paste-like viscosity. Thixotropic (non-sag) properties can be provided by incorporating precipitated silica or fumed silica into the compound.

    In certain applications including aerospace and automobile, very lightweight polyester compounds are required. In this case, glass or plastic microballons are substituted for a portion of the other inorganic components in the formulation.

    Should you have any comments or feedback, please contact me.

    Edward M. Petrie

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    SpecialChem4Adhesives Members Reactions

    very well - May 09, 2014
    posted by milan patel, Other Departments (Finance, IT, Legal, HR...) at personal
    this article is verrygood for me

    up - Feb 24, 2014
    posted by saeed oshaghi, R&D - Applied/ Formulation/ Product development at poly
    thanks a lot for detail.

    Single component unsaturated polyester - Mar 17, 2012
    posted by Atindra Sanyal, R&D - Applied/ Formulation/ Product development at CCEPL
    Is it possible to use unsaturated polyester resin in styrene solution as a single component coating? The resin solution may crosslink in presence of oxygen / moisture after application.

    polyester resin - Jan 12, 2012
    posted by Harry ZHU, R&D - Applied/ Formulation/ Product development at Huntsman
    any recommendation on polyester resin used in adhesive formula, how to choose?

    need more information in formulation - Mar 26, 2011
    posted by badri narayan, R&D - Applied/ Formulation/ Product development at rishabh consultancy
    styrene monomer diluent role with respective curing time. the use benzylperoxide as catalyst.

    - Oct 26, 2010
    posted by sankar raman, R&D - Applied/ Formulation/ Product development at IIT

    - Feb 21, 2010
    posted by khaled ramadan, R&D - Applied/ Formulation/ Product development at almadinamisurata

    Catalyst - Aug 08, 2009
    posted by joab rodas, Production / Manufacturing at merport e.i.r.l
    congratulations! very good explanation of an UPR and putty formulation,was important for me.
    what about formulation of a paste catalyst and the manufacturing process of the putty.could be very interesting.
    best regards: joab rodas

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