The Influence of Composition, Distillated and Synthetic Sea Water and Elevated Temperature on Mechanical and Physical Characteristics of Four Cured Different Polyester Systems

In this paper the influence of composition of resins, distillated and synthetic sea water on mechanical and physical characteristics of four different polyester resin (PR) systems is presented. The basic data about chemistry of PR, ingredients of PR systems (polyhydric alcohols, an unsaturated polybasic acids, a saturated polybasic acids and monomer), process for manufacturing of hardened and post cured specimens and testing procedure are presented. Specimens of four PR systems were exposed to the influence of distillated and synthetic sea water at 23 °C for a period of thirty days (the first experiment) and distillated boiling water (100 °C) for a period of 210 minutes (the second experiment). Hardness, impact resistance, water soluble matter and water absorption of specimens of four PR systems were recorded as the parameters that demonstrate the quality changes of tested materials in mentioned conditions of both experiments. It is established that ingredients and distillated and synthetic sea water have important influence on tested mechanical and physical characteristics of four PR systems.


The list of abbreviations used in the paper
Introduction ECOND half of the previous century and decades of the current century up to today are considered as a period of intensive utilization of polymeric materials, although initial steps in development were recorded in the nineteenth century. Polyester resins have been one of mostly used polymeric thermosetting material in a long period of time for manufacturing very dissimilar things [1].
By chemical definition a polyester is formed by the reaction of a polybasic acid and a polyhydric alcohol to form a series (poly=many) of ester linkages.
The first attempt to make a polyester resin or an alkyd is attributed to J.J. Berzelius (Sweden) who, in 1847, reacted tartaric acid with glycerol to obtain a resinous mass. As early as 1916, resinous compositions, based on the combination of glycerol and phthalic anhydride were used as finishes for wood and metal. The combination of fatty acids from vegetable oils with alkyds by R.H. Kienle (USA), in 1927, was a major step toward useful products similar in compositions used in many of modern finishes. Kienle is given credit for coining the word ALKYD (derived from ALcohol-aCID and spelled phonetically). In 1937, Carleton Ellis discovered that the addition of unsaturated monomers to polyesters containing unsaturated groups increased their setting speed by more than thirty times. The combination of curing speed and properties of the resulting polymers gave the first thermosetting polyester resin which can be put in commercial use. For his key discoveries in the development of polyesters C. Ellis has sometimes been called the "father" of unsaturated polyesters [2].

Chemistry of polyester resins
In inorganic chemistry reaction between an acid and a base (so called neutralization reaction) produces salt and water.
A similar reaction involving organic molecules (based on carbon backbone) would be a reaction of organic acid and alcohol ("organic base") to yield an ester. For example, acetic acid and ethyl alcohol, in so called esterification or S condensation reaction, produced ethyl acetate and water.
Since the involved molecules are monobasic acid and monohydric alcohol, they have only one reactive group (each) and reaction stops with the formation of a "simple ester" (Fig.1) [2]:

Ingredients of polyester resin system
Innumerable variations can be made in the composition of the base polyester and the type and amount of cross-linking monomer, to yield resins with a wide range of characteristics before, during and after polymerization.
To obtain a polymer chain which can be cross-linked into thermosetting resin, most often, a proportion of the dibasic acid must contain an unsaturated group or double bond. This unsaturation must be non-aromatic otherwise cross-linking will not take place. It is worth mentioned that styrene will react through the pendant vinyl group in the presence of suitable catalyst, whilst the benzene aromatic nucleus does not react.
The types of acid selected-with respect to their configuration, pendant groups, and presence of unsaturated bonds available for cross-linking, molecular weight-contribute to the properties of the final product. Most of polyester resins are made up of at least two dibasic acids (mix of unsaturated and saturated acids), whilst a single unsaturated resin is only used where a high degree of cross-linking is desired. By varying the ratio of the two anhydrides or acids, the crosslinking reactivity of the polyester resin can be varied from high to low. A high reactivity polyester resin contains a high proportion of unsaturation.
Such a formed polymer could be converted into a thermoset by addition of catalyst (invariably peroxide). In order to convert this polymeric material into a usable resin, a further component, which acts as solvent for the polymer chain (i.e. viscosity reducer of the resin) and cross-linking agent, is necessary. Its component is monomer, which, during cure links the polymer chains together through the unsaturated reactive groups, to give a usable product [3]. Thus, it can be seen that there are four main components needed to produce a usable resin, namely: (I) polyhydric alcohol, (II) unsaturated polybasic acid, (III) saturated polybasic acid and (IV) reactive monomer.

Polyhydric alcohol
Polyhydric alcohol component of the polyester structure has as important influence on the properties of the cross-linked copolymer as any of the acids used. The principal alcohols used in polyester synthesis are dihydric alcohols (glycols). Monohydric alcohols are sometimes used to determine the chain growth. Alcohols that contain more than two hydroxyl groups give branching in the polyester chain [2].
Diethylene glycol (IUPAC name 2,2'-Oxydi(ethan-1-ol)) is classed as dihydric alcohol [4]. DEG is a most common abbreviation for the diethylene glycol. DEG is produced by the partial hydrolysis of ethylene oxide and the resulting product is two ethylene glycol molecules joined by an ether bond [5].
Propylene glycol (IUPAC name: propane-1,2-diol) chemically is, also, classed as a diol, i.e. dihydric alcohol [4]. Propylene glycol (abbreviated PPG) is sometimes called αpropylene glycol to distinguish it from the isomer propane-1,3-diol (β-propylene glycol). It gives non-crystallizing polyester resin completely compatible with styrene. This glycol is relatively cheap and thus is used in the bulk of all polyester resin produced today.
Neopentyl glycol (IUPAC name: 2,2-dimethylpropane-1,3diol) is dihydric alcohol containing two primary hydroxyl groups [4]. It is synthesized industrially by the aldol reaction of formaldehyde and isobutyraldehyde [6]. Sometimes it is blended with other glycols to improve the styrene compatibility of the resin.
Pentaerythritol (IUPAC name 2,2-Bis (hydroxymethyl) propane-1,3-diol) is polyhydric alcohol with more than two hydroxyl groups [4]. The word pentaerythritol (abbreviated in a polyester resin terminology PET) is a linguistic blend of words penta-in reference to the number of carbon atoms and erythritol, which possesses 4 hydroxyl groups. This alcohol is used in the manufacture of polyester resins to introduce branching in the polyester chain.

Unsaturated polybasic acids
Unsaturated polybasic acid present in the polymer chain enables it to be cross-linked. The higher the proportion is present, the higher the degree of cross-linking. This component, also, affects the reactivity of resin. The higher the degree of unsaturation, the higher is the reactivity of the resin. The most commonly used unsaturated polybasic organic acid is maleic acid (melting point range from 132 °C to 140 °C), generally used as the anhydride (melting point 60 °C) due to its lower melting point.
Reactivity and cured properties of a polyester resins can be modified by blending unsaturated polybasic acids with saturated polybasic acids.

Saturated polybasic acids
The term saturated is used to describe those dibasic acids or anhydrides which do not contain pendant double bonds, which react in the present of the catalyst. They may contain aromatic nucleus.
The anhydride of orthophthalic acid (IUPAC name 2-Benzofuran-1,3-dione) was first suggested for use in polyester production [4]. This compound is often just called anhydride phthalic acid (abbreviated APA) because it is only isomer capable of forming an anhydride. Resins prepared with this anhydride are clear and have good compatibility with styrene. Since it is relatively cheap, easy available and since polyester resin prepared from it have good all-round properties, it has remained one of the most widely used anhydride.
The isophthalic acid (IUPAC name Benzene-1,3dicarboxylic acid) is the next most commonly used saturated acid, more expensive than APA, but it gives resins with higher molecular weight [4].
Skeletal formulae of anhydride of phthalic acid (a) and isophthalic acid (b) are presented in Fig.7.

Monomer
The monomer serves two purposes: (a) to act as solvent for а polyester resin to produce a liquid with a suitable handling viscosity and (b) to connect the polyester chains with double bonds to give fully cross-linked thermoset structure in copolymerization reaction.
Styrene (IUPAC name Ethenylbenzene) is characterized by low viscosity, low cost and ready availability [4]. The formula of the most frequently used monomer is presented in Fig.8. The other monomers are methyl methacrylate, dichlorostyrene and α-methyl styrene.
Other, frequently, used components in fabrication of polyester resin-based products is catalysts, accelerators and inhibitors [3].
Catalyst or initiators for unsaturated a polyester resin systems consist of organic peroxides which yield highly reactive free radical species [8]. Cure of unsaturated a polyester resin takes place in polymerization process through unsaturated groups both in polyester chain and the monomer, initiated by free radical. The free radicals are provided by the peroxide as it decomposes and it is the rate at which these free radicals are produced which governs the gel and cure time of the resin. Organic peroxides can be divided into two broad classes: the true peroxides (as benzoyl peroxide) and the hydroperoxides (as cumene hydroperoxides). The most commonly encountered peroxide catalyst for room temperature cure is methyl ethyl ketone peroxide (abbreviated MEK peroxide), which is considered to be mixed peroxide. MEK peroxide is very explosive in its pure form and is therefore always supplied diluted in plasticizer, usually dimethyl phthalate or dibutyl phthalate.
Accelerators or promoters are materials that are used in conjunction with an organic peroxide catalyst to increase the rate at which peroxide breaks down into free radicals. Thus, they accelerate the cure of the polyester resin in controlled manner. They can be divided into two classes of materialsmetal compounds and tertiary amines. Metal compounds are usually metal salts or soaps in solutions of different plasticizers. Cobalt salts are frequently and manganese and vanadium accelerators occasionally used. Cobalt naphthenate (diluted into styrene) is mostly used because it is excellent accelerator for hydroperoxides and mixed peroxides.
Inhibitors are added to ensure adequate shelf-life of resin. Some inhibitors increase gel time and cure time as well as pot-life, whilst others just increase pot-life without influencing two mentioned technological parameters. Two inhibitors which are used to prolong pot-life are t-butyl catechol and di-t-butyl-p-cresol.
The curing of a polyester resin can be considered to take place in three stages [3]: 1. Gelation -where resin changes from a free-flowing liquid to a soft gel. 2. Hardening -where the resin cures from a soft gel to a hard material capable of being removing from the mold. 3. Post cure or maturing -where the resin achieves its full mechanical, physical and chemical properties. For room temperature cured systems, post curing or final curing may be carried out at elevated temperature. This process should always be carried out immediately after hardening stage while there is still sufficient residual peroxide to complete cure. All resins absorb water to a greater or lesser extent. Water is, indeed, an important degradation factor for polyester-resin based polymeric products [8]. The absorption of a resin can be attributed to the moisture affinity of the highly polar functional group in the cured resin. In a polyester resin the higher molecular weight, the smaller will be the number of the hydrophilic end groups of the polyester ingredients [9].
Material characteristics are defined as measuring values, by which the shape or the measure of material capability to react to external influence are characterized. In accordance with the nature of external influences on a material, material characteristics can be classified into three groups: 1) mechanical characteristics, 2) physical characteristics and 3) chemical characteristics [10]. Mechanical properties refer to the material behavior under the influence of external mechanical forces. Physical properties refer to the material behavior under physical stress.
Water absorption content and quantity of water soluble matter are materials characteristics that reflect the quality changes of the all cured polyester resins during tested periods of time in mentioned liquids.
The effects of the water on the resin can be to cause plasticization process, swelling process and bond breaking process. It is stated that most of the changes, resulting from the expansion of the resin by water, occur in the vicinity of the surface of a polyester resin materials. Investigation of Barcol hardness belongs to so called shaped matter indention tests. Barcol hardness measures the resistance of the surface and nearby polyester resin material to penetration of sharp steel part under spring.
It can be supposed that some water pick-up effectively "anneals" polyester resin product. Impact resistance is measure of energy required to break a specimen and thus is property of the whole polyester resin based product. In other words, impact resistance is measure of the characteristic of total mass of the polyester resin system material.
Barcol hardness was chosen as an important parameter of the quality of a polyester resin system [11]. The same refers to impact resistance.
Eckstein, who examined over 70 dissimilar resin formulations, has concluded that water absorption may differ by a factor of 10 between various resin types and up to three for the same resin with a different curing agent [12].
Pritchard and Speake predicted that 3.6 % water would be absorbed by the cast unreinforced PR after ten years -this is about the same as after three days at 100 °C, thus emphasizing the severity of boiling water test [13].
Belan, Bellenger, Mortaigne and Verdu tested propylene glycol/neopentyl glycol-maleate/isophthalate copolymers crosslinked by styrene at various temperatures between 30 °C and 90 °C. They concluded that water concentration increases with the temperature and that material expansion is induced by temperature [14].
For the purpose of this paper, impact resistance and Barcol hardness, as a mechanical characteristics, and water absorption content and quantity of water soluble matter, as physical properties, are chosen to follow the influence of a polyester resin ingredients and distillated and synthetic sea water at laboratory temperature and distillated water at boiling temperature to quality of four tested polyester resin based systems.
In this paper the influence of ingredients of a polyester resin and the influence of water of laboratory and elevated temperature on mechanical and physical characteristics of cured four different polyester systems are presented.
Polyester resin system (abbreviated for the purpose of this paper -PR system) mark 1 consists of DEG/PPG/APA/AMK/St, PR system mark 2 of PET/PPG/APA/AMK/St, PR system mark 3 of PPG/IPA/AMK/St and PR system mark 4 of NPG/IPA/AMK/St.
Common properties of mentioned four PR system (visual appearance, viscosity, acid number and gel time) are presented in Table 1. Table 1. Common properties of PR system mark1, PR system mark 2, PR system mark 3 and PR system mark 4

Property
PR system mark 1 PR system mark 2 PR system mark 3

PR system mark 4
Visual appearance Using each of four mentioned PR system and the catalyst and an accelerator (applied for determination of gel time) a homogenized mixtures were produced and poured in metal mold dimensions 120 mm x 15 mm x 10 mm. Homogenized mixtures were allowed to hardened for 24 hours at 23 °C, afterwards the post curing procedure were applied (2 hours at 60 °C and 7 x 24 hours at 23 °C).
One part of specimen dimensions 120 mm x 15 mm x 10 mm is intended for determination of water absorption, watersoluble matter and impact resistance. Another part of hardened and post cured bars dimensions 120 mm x 15 mm x 10 mm of all four mentioned PR systems was used for production of specimen shape prism dimensions 25 mm x 15 mm x 10 mm by machining. The later specimens are intended for determination of water absorption, water-soluble matter and Barcol hardness.
These specimens are used for determining mechanical characteristics (impact resistance and Barcol hardness) and physical properties (water absorption and water soluble matter) before, during and after exposure to the influence of distillated water and synthetic sea water at 23 °C and distillated water at boiling temperature (100 °C) [15].
Barcol hardness is very simple, fast, and useful test used to determine the degree of cure as a parameter of PR system quality. Generally, a well fabricated, well cured product will have a minimum 30 Barcol degrees [16].
Unsaturated PR system is the most important polymers for production of casting resins (no reinforced) and composite (reinforced) materials [17]. These materials are used in many different areas, including those in which the products are exposed to the influence of water [18].
Although it is difficult to predict on the basis of laboratory data alone how any specific polyester product will stand up on exposure to a particular environment and chemicals, results, obtained by testing specimens in conditions similar to those in which produced part will be used, can be very helpful.
Specimens of four PR systems were exposed to the influence of water. Specimens of dimensions 25 mm x 15 mm x 10 mm and of dimensions 120 mm x 15 mm x 10 mm were exposed to the influence of distillated water and synthetic sea water at 23°C for a period of 30 days and distillated water at boiling temperature (100 °C) for a period of 210 minutes.

Results and analysis
In accordance with the procedure described in experimental part of this paper, appropriate sets of specimens dimensions 120 mm x 15 mm x 10 mm and appropriate sets of specimens dimensions 25 mm x 15 mm x 10 mm of all four mentioned PR systems were produced.
Four sets of specimens dimensions 120 mm x 15 mm x 10 mm of all four mentioned PR systems were immersed in distillated water at 23 °C for a period of thirty days. During mentioned period, after certain periods, specimens were removed from the water, treated in accordance with appropriate standards, and water absorption, water soluble matter and impact resistance were tested.
Four sets of specimens of all four mentioned PR systems, which have above mentioned dimensions, were immersed in synthetic sea water at 23 °C. This investigation lasted for a period of thirty days and after а while, specimens were removed from the mentioned liquid, treated in accordance with appropriate standards, and water absorption, water soluble matter and impact resistance were tested.
Two mentioned investigations were done, also, with appropriate number of specimens dimensions 25 mm x 15 mm x 10 mm of all four mentioned PR systems, i.e. four sets of this specimens were immersed in distillated water at 23 °C for a period of thirty days and another four sets of specimens of same dimensions were immersed in synthetic sea water at 23 °C for the same period of time. During mentioned time, after certain periods, the specimens were removed from the appropriate water, treated in accordance with chosen standards, and water absorption, water soluble matter and Barcol hardness were tested.
Specimens of dimensions 25 mm x 15 mm x 10 mm of all four mentioned PR systems were exposed to the influence of boiling water for a period of 210 minutes and after every thirty minutes, specimens were removed, treated as it is defined in adequate standards, and water absorption, water soluble matter and Barcol hardness were tested.
Set of specimens dimensions 120 mm x 15 mm x 10 mm of all four mentioned PR systems were immersed in the boiling water for a period of 210 minutes. During this period, after every thirty minutes, specimens were removed from the boiling water, treated as it is defined in appropriate standards, and water absorption, water soluble matter and impact resistance were tested.
Water absorption and water soluble matter were tested in accordance with ISO 62 standard [20]. Impact resistance was determined following the instructions in ISO 179 standard [21].
Testing Barcol hardness was done in a line with ASTM D 2583-13a standard [22].
Viscosity, acid number and gel time were tested in accordance with ASTM D 1200, ISO 2114 and ISO 2535 standards, respectively [23,24,25].

Testing results with distillated and synthetic sea water and analysis
One specimen dimensions 25 mm x 15 mm x 10 mm (abbreviated for a table presentation in this paper-H.S.hardness specimen) and one specimen dimensions 120 mm x 15 mm x 10 mm (shortened for a table presentation in this paper -I.R.S. -impact resistance specimen) of each of four polyester resin systems were immersed in distillated water and synthetic sea water at 23°C for a period of thirty days. Data recorded during this testing and calculated corresponding water absorption (single value) and water soluble matter (single value) are presented in Table 2. Single values (x i ) and arithmetic mean value ( x ± δ) for water absorption of specimens marks H.S. and I.R.S. of all four mentioned PR systems in distillated water at 23 °C for the periods of 5, 10, 20 and 30 days are presented in Table 3.
Arithmetic mean value is based on three single values. This refers to all arithmetic mean values in this paper.  Fig.9.  Table 4.  for all four PR systems in synthetic sea water at 23 °C for a period of thirty days are presented in Fig.10. Single values (x i ) and arithmetic mean value ( x ± δ) for water soluble matter of specimens marks H.S. and I.R.S. of all four mentioned PR systems in distillated water at 23 °C for a period of thirty days are presented in Table 5.  S. (b)] for all four PR systems in distillated water at 23 °C for a period of thirty days is presented in Figure 11. of all four mentioned PR systems in synthetic sea water at 23 °C for a period of thirty days are presented in Table 6.    Table 7. As can be seen in Table 7, the longest drying time showed PR system mark 1 specimen, which is in accordance with the results of water absorption and water soluble matter of mentioned system. The shortest drying time showed PR system mark 4 specimens, also, based on values of two mentioned tested characteristics.
PR system mark 4 has the greatest resistance toward distillated and synthetic sea water due to basic components, including alcohol (neopenthyl glycol) and saturated acid (isophthalic acid) and synthesis procedure (the highest molecular weight of all tested polyester resin systems). The positive influence of neopentyl glycol to hydrolytic stability of PR system mark 4 is based on a fact that two H atoms of αcarbon atom are replaced by two methyl groups. The greatest water resistances of PR system mark 4 means that this PR system showed the smallest both water absorption and water soluble matter.
From the practical point of view the worst water absorption value and water soluble matter value of four tested PR systems is recorded at PR system mark 1. This phenomenon is caused by influence of diethylene glycol (because ether linkages in this constituent have increased water and heat sensitivity) and of ortho phthalic acid (which produces resin of lower molecular weight regarding isophthalic acid).
By the analysis of data presented in Tables 2 to 7 and Figures 9 to 12, which refers to water absorption and water soluble matter in distillated water and synthetic sea water, it can be seen that the results obtained by testing hardness specimens and impact resistance specimens are similar. This fact indicates that the procedure of manufacturing of mentioned specimens and procedure of testing hardness specimens and impact resistance specimens, after immersing in distillated water and synthetic sea water, were done correctly.
Barcol hardness [single values (x i ) and adopted value for later analysis], expressed in Barcol degree (abbreviated for a table presentation in this paper-B.D.) of H.S. of each of four PR systems, after immersing in distillated water and synthetic sea water at 23 °C for a periods 5, 10, 20 and 30 days, are presented in Table 8. Barcol hardness (adopted values) of H.S. of four PR systems, after immersing in distillated water (a) and synthetic sea water (b) at 23 °C for the periods of 5, 10, 20 and 30 days, are presented in Fig.13. All four PR systems have similar initial Barcol hardness. By analyzing Barcol hardness data, obtained after the influence of distillated and synthetic sea water at the laboratory temperature (23 °C) for a period of 30 days, it can be seen, nominally, that highest decrease is shown by PS system mark 1 (from 20 % to 25 %). PR systems marks 2 and 3 have similar Barcol hardness drop (about 7 %) and PS system mark 4 something higher (about 10 %).
Based on Barcol hardness results for PR system mark 2, PR system mark 3 and PR system mark 4, after immersion in distillated and synthetic sea water for 5, 10, 20 and 30 days and for PR system mark 1, after immersion in distillated and synthetic sea water for 5 days, one may conclude that a plasticization process started slightly. Important drop of Barcol hardness for PR system mark 1, after immersion in distillated and synthetic sea water for 10, 20 and 30 days, can be an indication of beginning of swelling and breaking bonds processes in surface layer.
Impact resistance [single values (x i ) and arithmetic mean value ( x ± δ)] of I.R.S. of each of four PR systems, after immersing in distillated water and synthetic sea water at 23 °C for periods of 5, 10, 20 and 30 days, is presented in Table 9. Impact resistance of I.R.S. of four PR systems, after immersing in distillated water (a) and synthetic sea water (b) at 23 °C for the periods of 5, 10, 20 and 30 days, is presented in Fig.14. (а) (b) Figure 14. Impact resistance of I.R.S. of four PR systems, after immersing in distillated water (a) and synthetic sea water (b) at 23 °C for the periods of 5, 10, 20 and 30 days Impact resistance results can be broadly divided into two groups: -in the first are PR systems marks 2 and 4, whose initial impact resistance is 16.79 and 14.92 kJ/m 2 , respectively and -in the second are PR systems marks 3 and 1, whose initial impact resistance is 6.99 and 5.11 kJ/m 2 , respectively. PR systems from the first group have branched alcohol in their composition.
PR system mark 2 has pentaerytritol, which introduces branching in the polyester chain and improves mentioned physical property and resistance toward distillated and synthetic sea water. High molecular weight of this system, also, has a positive influence to the tested characteristics.
Neopentyl glycol, present in PR system mark 4, primarily provides high impact resistance and, obviously, improves water resistance although isophthalic acid and high molecular weight, also, have positive influence on the mentioned characteristics.

Testing results with boiling water and analysis
One specimen dimensions 25 mm x 15 mm x 10 mm and one specimen dimensions 120 mm x 15 mm x 10 mm of each of four PR systems were immersed in boiling distillated water for a period of 210 minutes. Data recorded during this testing and calculated corresponding water absorption (single value) and water soluble matter (single value), are presented in Table 10.  Table 11.  for all four PR systems in boiling water for a period up to 210 minutes is presented in Fig.15.  Table 12. In Figure  One specimen H.S. of each of four PR systems was immersed in boiling water for up to 210 minutes. Data recorded during drying these specimens to constant mass, after mentioned immersion, are presented in Table 13.  Table 13 are similar to those presented in Table 7 regarding drying time of specimen to constant mass.
PR system mark 1 showed the longest drying time of a specimen because this system had the highest both water absorption value and water soluble matter value.
PR system mark 4 has the smallest both water absorption value and water soluble matter value and the smallest drying time was necessary to obtain specimen of constant mass.
Barcol hardness of H.S. of each of four PR systems, [single values (x i ) and adopted value for later analysis], expressed in Barcol degree, after immersing in boiling water for the periods up to 210 minutes, are presented in Table 14. Barcol hardness (adopted values) of H.S. of four PR systems, after immersing in boiling water for the periods up to the 210 minutes, are presented in Fig.17. Changes in Barcol hardness after immersion in boiling water are similar to those after the influence of distillated and synthetic sea water, but in greater extent. PS systems mark 2 and 3 have similar drop of Barcol hardness (about 13 %) and PS system mark 4 had something higher decreasing of this characteristics (about 17 %). The highest drop of this property is recorded at PS system mark 1 (about 30 %) and it is important to emphasized that Barcol hardness after immersing in boiling water for the period of the 210 minutes is at the lowest acceptable level for correctly manufactured polyester resin product (30 Barcol degrees).
Specimens I.R.S. of each of four PR systems were immersed in boiling water for a period up to 210 minutes. Impact resistance [single values (x i ) and arithmetic mean value ( x ± δ)] of these specimens, after mentioned immersion, are presented in Table 15. Impact resistance of I.R.S. of four PR systems, after immersing in boiling water for the period up to the 210 minutes, is presented in Fig.18. After exposure to severe conditions in boiling water for 210 minutes PR system mark 2 and PR system mark 3 showed the smallest, mutual almost the same, diminution of impact resistance (about 20 %). Somewhat higher decrease of this characteristic was observed at PR system 4 (about 40 %). The highest reduction of impact resistance was recorded at PR system mark 1 (about 60 %), and this fact represents another argument that this system has higher sensitivity to water, especially boiling.
Similar results of water absorption and water soluble matter in boiling distillated water, obtained by testing hardness specimens and impact resistance specimens (Tables  10 to 15 and Figures 13 to 18) pointed out that procedures for producing and investigating of both specimens have been done correctly.

Conclusions
From the all presented statements and results, it can be concluded: 1. Based on the composition and obtained laboratory results of polyester resin system NPG/IPA/AMK/St, it can be concluded that neopentyl glycol has a positive effect on tested physical and mechanical characteristics. 2. Polyester resin system DEG/PPG/APA/AMK/St showed the smallest water resistance. Analyzing the composition of the mentioned system, this fact can be attributed to the sensitivity of ether linkages of diethylene glycol, primarily, and to a smaller molecular mass, regarding the orthophthalic acid. 3. Branched alcohols have a positive influence on impact resistance of tested polyester resin systems as well as high molecular weight. 4. All four polyester resin systems have similar initial Barcol hardness and, excluded polyester resin system DEG/PPG/APA/AMK/St, showed approximately similar behavior in tested environments. 5. Boiling water caused higher drop of tested properties of used polyester resin systems than distillated and synthetic sea water at 23°C. This is extremely obvious regarding the Barcol hardness of polyester resin system DEG/PPG/APA/AMK/St. 6. Synthetic sea water has slightly less effects on tested physical and mechanical characteristics of four cured polyester resin systems than distillated water.