Greater Material Flexibility For Metal Replacement Strategies

With the change in the DNA of copolymer POM, a copolymer has been developed with the mechanics properties of a homopolymer.

Many readers of this material column may remember last year’s Take Advantage of Emerging Megatrends post in which our global R&D director wrote about pushing material innovation back into the reactor to change the backbone chemistry of an engineering polymer that will meet customer applications requirements.

We changed the DNA of Hostaform POM, the Ticona brand name for polyacetal copolymer (POM), to create a truly innovative polymer in the lab — a new high-strength performance member of the family of Hostaform POM Copolymer Series of grades that include high productivity and high impact performance.Gronner

Since its introduction in 2010, several studies have shown that the new Hostaform HS15 essentially offers the same mechanical properties as high strength homopolymer while maintaining the thermal stability and chemical resistance of POM copolymers. The results of these studies were presented in a Ticona paper titled A New High Strength POM Copolymer during ANTEC 2011 in Boston this May.

Long History in Engineered Replacements for Metal Parts

Injection molded POM homopolymers and copolymershave a 50-year history in engineered replacements for metal parts, including gears, cams, pulleys, connectors, handles, reservoirs, snap-fit parts, conveyor belt sections, and various other parts that require a balance of strength and toughness, as well as non-acidic chemical resistance, inherent lubricity, dimensional stability and creep resistance.

Different manufacturing processes are used to produce the homopolymer,(AcO(CH2O)mAc) and copolymer, (RO[(CH2O)mCH2CH2O(CH2O)n]pCH2OR) versions of POM. The homopolymer is produced via anionic polymerization of formaldehyde followed by ester end-capping of the thermally unstable end groups. Copolymer production involves cationic ring-opening polymerization of trioxane (cyclic trimer of formaldehyde) plus a small amount of comonomer and chain-transfer agent which produces stable ether end groups in the same single step. These differences allow some flexibility of the structural details of the copolymer.

Although the differences in structural details are small, they lead to significant differences in the mechanical and chemical properties of the commercial resins: POM homopolymers provide higher strength, higher stiffness and higher impact resistance; while POM copolymers provide higher thermal stability and higher chemical resistance to alkaline media and hot water.

Historically, POM homopolymer producers have used stabilizers and additives to achieve the thermal stability of POM copolymers, and the POM copolymer producers have used modifications to achieve the mechanical properties of POM homopolymers.

Traditional Copolymer with Mechanics of Homopolymer

Now, with the change in the DNA of copolymer POM, a copolymer has been developed with the mechanics properties of a homopolymer:

  • Thermal and chemical stability of copolymer
    • Excellent chemical resistance to organic solvents, fuels, lubricants, alkalis and hot water (only copolymers) pH 4 to 14.
    • Higher melt stability, lower Formaldehyde (HCHO) emissions.
    • Excellent long-term heat aging (LTHA) performance.
  • Mechanical properties equivalent to homopolymer.
    • High toughness down to -40ºC.
    • High strength and stiffness.
    • High rigidity and resilience (creep resistance).
    • Good heat deflection resistance — operating up to 100ºC.
  • Low water absorption.
  • High dimensional stability.
  • Low coefficient of friction (wear, sliding characteristics).
  • Good electrical and dielectric properties.

As you can see, the new high strength POM copolymer grade combines the mechanical properties traditionally found inhigh strength POM homopolymers with the thermal and chemical (hot water and caustic) stability associated with traditional POM copolymers.

This combination of unique advantages is giving injection molders a greater flexibility in their choice of materials for metal replacement applications.

For additional technical information about this topic presented at ANTEC 2011, read the Ticona paper titled A New High Strength POM Copolymer.