Syntetic resins

Synthetic resins are also called polymers. Chemical structure of polymer molecules resemble a long chain consisting of many simple and repeated structural units typically connected by covalent chemical bonds. Polymers are often classified into two main groups depending on their response to temperature; thermohardening and thermoplastic polymers. Thermoplastic polymers (for example nylon, polypropylene, ABS resin) soften or melt while being heated up (like metals) and harden again in lower temperatures.

Thermohardening polymers take solid form as a result of chemical reaction in situ: resin and catalyst (a substance causing chemical reaction but not joining into molecules with reacting substance) or a hardening agent (a substance causing a reaction and joining with basic reacting substance) after mixing undergo irreversible chemical reaction, thus form a hard product. Before the mixing process with a catalyst or a hardening agent resins have a form of semi-fluid mass like natural resins.

After a chemical reaction started with catalyst/hardening agent mixing, semi-fluid resins reach (after some time - regulated with the amount and/or a kind of a catalyst/hardening agent) a composition which makes it stop flowing and become a solid product ? this is a gel point. The time which passes from the beginning of the reaction until the gelation point is achieved is called gel time. However some time must pass until the resin finally hardens and takes its mechanical properties ? this is called hardening time. Sometimes resins are subject to additional elevated temperature hardening (for example 5 hours in the temperature of 80 C) in order to improve some mechanical properties. As the forming process is completed, the product remains hard even when heated up, although above some temperature (called the temperature of glass transposition -Tg) its mechanical properties can be changed creating a more elastic product, therefore the stiffness coefficient will decrease and finally the compressive and tensile force will be changed. Precooling below Tg restores previous mechanical properties.

There are a lot of various resin types, however majority of them is used for production of structural elements. These are poly-ester, vinyl-ester and epoxy resins. Among remaining resins the following are worth mentioning: phenol resins (used in places where greater flame resistance is needed), cyanoesters (with great dielectric properties), polyurethanes (with great hardness), bismaleimides (resistant to high temperature and used in aircraft industry) and polyimides (with greater resistance to high temperature and used for the production of aircraft engines or rocket missiles; they are expensive - one ton of this raw material costs over 120 000 USD).

Polyester resins are used most often among most popular ester and epoxy resins because of the price and application ease, among other in the construction of vessels, boats, etc. Polyester resins inside of the molecular chain have many ester groups (CO-O-C) and make it more susceptible to hydrolysis process than two remaining resins and inside of reactive elements chain (C*=C*) following the reaction with the catalyst form a structure of numerous cross links and as a result give lesser parameters of mechanical resistance than the two remaining resin types.

Polyester resins have limited storage time (shelf time) as after a long period of time (many months) they undergo a gelation process even without the addition of a catalyst. A catalyst is added to make the gelation time practically useful.

Besides the catalyst also another agents can be added: agents regulating the reaction time, pigments, filling materials, chemical- and fire-resistance enhancing agents, depending on required applications and user requirements. Individual elements must be properly selected also in terms of quantity; too much of a catalyst would reduce the gelation time, too little ? would excessively prolong it. Most often styrene is added as a catalyst.

Vinylester resins have a lower number of ester groups which makes them less hydrolysable and active parts are located at the ends of the molecular chain which, after hardening, makes the structure more mechanically resistant than polyester resins. For example, tensile force for polyester and vinylester resins is ca. 5 MPa at 7 days of hardening in room temperature, but after additional overheating for 5 hours in 80 C this ratio is ca. 6.5 MPa for polyester resin and ca. 8 MPa for vinylester resin. The tensile modulus after 7 days of hardening in a room temperature of 20 C for polyester resin is ca. 2 GPa and ca. 2.5 GPa for vinylester resin. Additional overheating for 5 hours in 80 C improves these ratios for both resin types (ca. 2.9 GPa for polyester and ca. 3 GPa for vinylester resin).

During the chemical reaction after mixing the ester resin with a catalyst a significant transposition of the molecular structure takes place, which in result causes (among others) a contraction effect, even up to 8%.

Epoxide resins have higher ratios of mechanical properties (tensile force ca. 6 MPa after 7 days in temperature 20 C and ca. 8 MPa after 5 hours in temperature of 80 C, tensile modulus ca. 3.4 GPa), higher chemical and thermal resistance and also better adhesiveness (ca. 2000 p.s.i. in comparison to 500 p.s.i. for vinylester resin) and exceptionally good water resistance.

The name ?epoxide? relates to the group consisting of oxygen atom which is located in the molecule of this chemical resin, bonded with two carbon atoms, which have already been somehow bonded to each other.

Epoxide resins, called epoxides, polyepoxides or epoxide polymers are usually obtained in result of a reaction between epichlorohydrin and bisphenol A (both these agents, and in particular epichlorohydrin, are unfortunately harmful and it is the ready-made resin that is considered as an almost non-hazardous product, although it is irritating, however after hardening the epoxide resin is a non-hazardous product. It is however advised not to inhale dust produced during its mechanical processing). Amine is most often used as a hardener.

The discoverers in this area are Dr Pierre Castan from Switzerland (whose works were sponsored by the Swiss company Ciba, and in the 1990s this company?s epoxide production was bought by Huntsman from USA) and Dr S.O. Greenlee from USA, working for Devoe-Reynolds (which was sold to Shell Chemical, then to Resolution Polymers, currently Hexion). Currently the world business in epoxide area is estimated at ca. 15 billion USD, of which over 30% constitutes the sales of the basic raw material, of which the world scale manufacturers are the following 3 companies: Hexion (produces resins under trade name ?Epon?), Dow Chemical (produces ?D.E.R.?) and Huntsman (produces ?Araldite?). Besides this there are a few dozens of local manufacturers, manufacturing special resins, hardeners, modifiers etc. The remaining 60% of the market belong to companies referred to as ?formulators?, which develop hundreds of chemical formulas adjusted to the needs of individual branches or users (usually focusing on a given branch or group of users) and which, on the basis of basic raw materials, manufacture specific types of resins adapted to specified practical applications. It can be said that each of these companies applies a specific recipe and even if the products are based on the same basic raw material, the final product can be very different in terms of its properties.

The popularity of resin applications increases in construction industry, as well as in many other fields of economy, because it is possible to develop a recipe which meets exactly the individual construction and mounting requirements. Depending on their application ? as a filling, adhesive, heavy-load resistant material or suitable for specific mounting technology, a proper recipe is developed.

Resins used as a bonding element in construction and mounting works as well as heavy-load transferring material, are mainly ester and epoxide resins and different types of mixtures containing epoxide, acrylate and vinylester resins.

A chemical bonding system adjusted to construction and mounting requirements and able to convey heavy loads consists of using the resin properties, i.e. high mechanical parameters after hardening, their plasticity, ability to shape resins before hardening and their adhesiveness to almost any material and almost in every conditions. Many different elements are added to resins, such as pigments, reaction-time reducing or accelerating agents, filling materials etc. and depending on a recipe (which can be easily altered) a product is obtained ? suitable for the customer?s requirements (mechanical parameters, simplicity of application, adjustment to use in different conditions etc.). Usually it is not possible to obtain a recipe improving all possible qualities and improvement of some parameters is done at the cost of deteriorating other parameters, which must also be taken into account in the marketing strategy. For example: simplicity of application does not go along with quality and bonding force, better adaptation for bonding in case of a given surface always deteriorates its versatility and bonding on other surfaces, bonding in low temperatures does not go along with the simplicity of application in high temperatures and so on. The marketing strategy and the recipe are strongly related to one another.