isophthalic resin...

Discussion in 'Materials' started by Lew Morris, Jan 8, 2003.

  1. Lew Morris
    Joined: Jun 2001
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    Lew Morris Industrial Designer

    i admit it, i'm not the sharpest knife in the drawer... what is/are isophthalic polyester resins?

    this is as far as i've gotten,

    iso-, 1. a learned borrowing from Greek meaning"equal", used in the formulation of compound words: isochromatic. 2. Chem. a prefix added to the name of one compound to denote another isomeric with it....

    grrrrrrrrrrrr... we were taught in fourth grade english class not to define a term with itself... and there is NO definition for "phthalic".

    so, what are they used for? ... and why?

  2. stephan shugart
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    stephan shugart Junior Member

    I've gone to the materials section of and looked up manufactures and have found plenty of decriptive tecnical mombo jumbo on the subjects. Have fun I did.
  3. Guest

    Guest Guest

    Isophthalic resin refer to the aromatic dibasic acid used to make a polyester resin (other components are aliphatic acids, polyols, etc.). The "iso" term refers to the molecule arrangement. Basically this acid (or anhydride if not in the presene of water) is a benzene ring (a ring of 6 connected carbon atoms with attached hydrogen atoms) with two "arms" that are the crosslinkers in the condensation reaction that makes polyester resins. When these "arms"are connected to different carbon atoms in the benzene ring, they are denoted with a prefix (iso, tere, or ortho). When the arms are on two carbon atoms right next to each other, it's an orthophthalic, if they have one carbon atom inbetween they're isophthalic, etc., etc. Well, that's all the chemistry that I can remember...

    Isophthalics will have better mechanical and corrosive properties, as well as better elevated temp properties than ortho resins.
  4. Lew Morris
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    Lew Morris Industrial Designer


    Thanks for the basic information; improved mechnical, corrosion-resistance, and thermal properties. Sounds like they might be useful in large-part mold fabrication... less mold expansion during exotherm, resulting in a more dimensionally accurate laminated part (?).

    Mr. Shugart,

    I went back through all of the previous 'Materials Forum' postings looking for information regarding isophthalic resins before I posted my question. There were none. And I too checked the 'Materials' section before posting my question. It did not yield any useful manufacturer's information (that I could find) regarding the technical characteristics of isophthalic resins.

    Since this forum is essentially about sharing information, and "... hav(ing) found plenty of decriptive tecnical mombo jumbo on the subjects"... might you, perhaps, share your sources with me?
  5. Guest

    Guest Guest

    Most polyesters are not made from aromatic molecules . The exceptions [ that I know of ] are DCPD resins used for low shrinkage applications and some gelcoats .

    Polyester polymers are cooked from primarily Maleic Acid [or too save having to remove one water molecule for every polymerisation reaction maleic anhydride ] and a combination of di-basic glycols ie ethylene glycol , propylene glycol .

    The essential difference between iso and ortho is the particular isomer of maleic acid . Either the bonding sites at the end of the molecule [ that will hold hands with the glycols to form the polyester chains ] are adjacent OR they sit diagonally across the molecule .

    The failure of resin and fibre structures occurs when the bond between those two components is broken . There is a basic imbalance between the elongation of a cheaper ortho resin and glass fibre . These resins fail at about 2% elongation , well before the glass has reached the ammount of stretch that will break it, and thus the laminate fails .
    Iso polyesters stretch nearly as much as E glass and thus for the same weight of fibre the laminate will be significantly stronger .
    DCPD resins fail at about 0.5% elongation. They are give a prettier finish but are pretty hopeless for much else .

    To confuse the issue slightly most ortho laminates are fine because by the time a poly/glass laminate has become thick enough to reached sufficient STIFFNESS it is already far stronger than it needs to be . Hence cheaper and generally easier to use orthos are the predominant resin .
    Isos have their place in high load applications and in regard to chemical [ yes even salt water is a chemical ] resistance .

    We regularly build moulds of ortho . Our resin suppliers reccomend iso but we have production moulds that are 800 pulls old and are showing no deterioration .
  6. Lew Morris
    Joined: Jun 2001
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    Lew Morris Industrial Designer

    Guest #2,

    How big are your moulds? Do you notice any significant dimensional problems with the laminated parts produced using the ortho resins? It may not be a concern for you if everything is produced from moulds fabricated from the same materials. I've built molds (oops, my north american is showing) for boats (under fifteen feet) using ortho resin, but the other molds, for the other components in the suite, were also ortho. The laminated parts were ortho as well.

    I'm bidding on some (dare I say it here...) automotive body components (front and rear facia, and side skirts). They have to fit steel-bodied production vehicles so accurate mechanical reproduction is important. I'm starting with another manufacturers molded FRP part (for the basic mating surfaces) and will then re-model the part to my client's preference. I'm concerned about the difference in skrinkage between the molded part (used as a pattern) and the second generation mold, and finally the producion part, building up on me with a resultant no-go fit situation. THAT was wordy wasn't it... Basically I need to minimize shrinkage. Unfortunately epoxy is out of the running; I've never used it in a production situation, and don't want to learn at my client's expense.

    Can you tell me why iso's are preferred for tooling? Is it strictly for added strength-to-weight in a tool, or might it be shrink related? You mentioned 'easier to use...' How so? Do orthos tend to wet-out faster than isophthalic resins? Or is it...?

    I think the ortho resins will be fine for the production parts, it's mostly concerns regarding tooling that I have.

    So MANY questions....

    By the way, thanks for sharing.
  7. Mike D
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    Mike D Senior Member

  8. Guest

    Guest Guest

    Hi Lew .

    And thanks Mike for the excellent link .

    Im in New Zealand and regularly read the CFA magazine . It had never occured to me to look at the website . The magazines are an excellent source of information and I expect to learn even more via that site .

    We make automotive parts as well as sanitary ware and kayaks .
    We control shrinkage and distortion in the auto parts by postcuring each part in the mould . Our spray/bake painting oven runs at 60 C [ about 150 F ]
    Each night the moulds are wheeled into the oven which is then turned off . The next morning the residual heat is still about 30 C [90F ] .
    It helps a lot to eliminate unwanted distortion and shrinkage . As a bonus we have a preheated mould to gelcoat on in cold mornings .
    All our tooling is made with vynil ester tooling gelcoats to help handle the heat . Recently a salesman tried to sell me a "new improved" polyester tooling gelcoat which may well be a wonderful product but it didnt have the heat distortion temp of the VE
    Our conditions here ,and certainly our environmental laws are substantially different to the USA . Your local resins may be formulated to meet emmission requirements rather than say ease of use . Probably contacting the technical help of your resin manufacturer is the best option . Im sure they will know what products are used in your environment etc .
    I really doubt you need iso resins for your componentry and good mould making technique should keep shrinkage and distortion minimal
  9. tgundberg
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    tgundberg Junior Member

    Polyesters made without aromatics?? True, maleic acid (or anhydride without water) is used extensively in almost all polyester resins, but aromatic diacids are also used, and are used to name the type of resin. The aromtic diacids hydrolize more slowly than the maleic (a alphatic, or linear molecule), and without aromatic acids, the glass transition temperature would be extremely low.
  10. Lew Morris
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    Lew Morris Industrial Designer


    Ditto our kiwi friend (re: the CFA link). A great help.

    right now I'm wishin' i was in the spinnaker pole splint business

  11. stephan shugart
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    stephan shugart Junior Member

    Dear Lew,

    Please go to the, Materials section and click on the fiberglass and resin section. Then go to the Fiberglass Coatings Inc. selection. From there go to the Catalog section and pick polyesters.

    The list is quite extensive. decriptions and prices of all their products. They also have a tecnical link or just call them and they will send you all the tecnical data you need on the various resins along with samples if you need them. Don't be shy they are there to help.

    Lew if it would be of any additional help please go to They even have a forum. It is a wonderful find I think you will be pleased.

    Thanks Stephan
    Last edited: Jan 23, 2003
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