FIELD OF THE INVENTION

The invention relates to polyamides based on renewable, long chain dicarboxylic acids and diamines, and a method for producing the same. BACKGROUND OF THE INVENTION

Polyamides, better known under the generic name 'nylons' are a major class of engineering thermoplastics. They show excellent properties, such as high strength, flexibility and toughness, relative high melting points, good heat resistance and abrasion resistance, and chemical inertness. The major drawback of the polyamides is their ability to absorb moisture which has a detrimental influence on dimensional stability as well as mechanical, chemical and physical properties. Another feature of commercially-produced polyamides is their relatively low molecular weight (-10,000-30,000 g-mol" ¹ ).

Typically, polyamides are prepared via a polycondensation reaction in which diamine and dicarboxylic acid groups react to form a polymer linked through amide linkages, releasing water as a by-product. The amine group and the carboxylic acid group can be present as separate monomers (namely, as diamine and dicarboxylic acid molecules) or within the same, single monomer molecule.

Production of two of the most common types of polyamide, nylon 6 and nylon 6,6, reached 7.2 million tons in 2014. The applications of polyamides are broad and varied; ranging from automotive components, electronic products and coatings to filaments, yarns, packaging, sports equipment and appliances. Therefore, the demand and value of polyamides as a polymer is high and expected to increase. However, current annual production is primarily derived from petrochemical feedstocks. Demand for suitable bio-based monomer alternatives and renewable production on the industrial scale is growing, from both public consumers and industry. Furthermore, bulk of the commercial polyamide market is dominated by short-chain polyamides (namely, containing less than 10 carbons per repeating unit). These shorter-chained polyamides exhibit poor water stability and gas permeability properties, while their low number average molecular weights (Mn) (~10,000-30,000 g-mol ¹ ) further limit optimal material properties and performance. Therefore, in order to meet these growing demands, while also addressing aforementioned material drawbacks (poor moisture stability and low molecular weight range), novel pathways for the production of long-chain aliphatic polyamides are required.

US 2011/0165359 Al discloses a polyamide comprising at least two units having the formula X.Y, wherein X is an alkylaromatic diamine and Y is an aliphatic carboxylic diacid selected from dodecanedioic (C12) acid, tetradecane- dioic (C14) acid and hexadecanedioic (C16) acid. The carboxylic diacid comprises organic carbon from a renewable source determined according to standard ASTM D6866. The polyamide is prepared by polycondensation of the diacid with the alkylaromatic diamine.

US 2011/0111154 Al discloses a polyamide comprising at least one repeat unit having the formula X.Y, wherein X is a cycloaliphatic diamine and Y is an aliphatic dicarboxylic acid chosen from dodecanedioic (C12) acid, tetrade- canedioic (C14) acid and hexadecanedioic (C16) acid. The dicarboxylic acid comprises organic carbon of renewable origin determined according to standard ASTM D6866. The polyamide is prepared by polycondensation of the diacid with the cycloaliphatic diamine.

Ehrenstein et al. (2000, Polymer, 41, 3531-3539) discloses the preparation of polyamides with long alkane segments. Tetracosanedioic acid was utilised as a commercially available dicarboxylic acid, while tetratricontanedioic acid was synthesised via 'chain extending' cycloaddition, ring-opening and reduction steps. Nylon salt preparation and polycondensation were subsequently utilised to yield PA 6,24 and PA 6,34.

Ehrenstein et al. (2003, Macromolecular Chemistry and Physics, 204, 1599-1606) discloses the preparation of long chain aliphatic polyamides PA 2,34, PA 4,34, PA 8,34, PA 10,34 and PA 12,34. The process involves the synthesis of tetratricontanedioic acid to provide a longer dicarboxylic acid segment, followed by polycondensation.

The present invention provides novel long-chain polyamides having a number average molecular weight of at least 37,000 g-mol ^-1 with improved prop- erties, and a method for the production thereof.

DEFINITIONS

In the present invention,

the term "polyamide of the invention" refers both to the polyamide of the invention as such and the polyamide prepared by the method of the invention; the term "long chain polyamide" means that the polyamide comprises dicarboxylic acid and/or diamine monomers having at least ten carbon atoms in their carbon chain;

the term "homopolymer" means that each repeating unit of formula I in the polyamide is identical to each other;

the term "copolymer" means that there are two or more different repeating units of formula I in the polyamide;

the term 'nylon salt' means a crystalline solid which is obtained from the reaction between the dicarboxylic acid and diamine (base) prior to the poly- condensation reaction;

the term 'renewable sources' refers to origin from biomass, namely of from plants, animals or microorganisms, or biowaste and is different from fossil sources, which are derived from the organic remains of prehistoric microorganisms, plants and animals. BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide polyamides which contain long hydrocarbon segments in their structure. The long hydrocarbon segments are derived from dicarboxylic acids and/or diamines, preferably obtained from renewable sources.

Another object of the invention is to provide a method for preparing polyamides, consisting of a 'nylon salt' formation and filtration process, followed by a polycondensation step. The 'nylon salt' is formed between the long-chain aliphatic dicarboxylic acid and diamine. This method allows a high degree of stoichiometric control during polymerisation, producing polyamides with high purity, number average high molecular weight of at least 37,000 g-mol ^"1 (determined by proton NMR method) and narrow polydispersity in high mass yields.

In an aspect, the invention provides use of the polyamides of the invention for a broad range of applications, including but not limited to films and coatings, food packaging films, furniture, appliances, sports equipment, consumer goods, wire and cable, and automotive components.

The long chain polyamides of the invention have higher number average molecular weight and longer repeating units than conventional polyamides, providing certain advantages. Higher number average molecular weights result in superior mechanical properties, including tensile modulus, tensile strength, work to fracture, yield strength, elongation at break. Additionally, the combination of high molecular weight and lower melting points may be beneficial in coprocessing of the polyamides with other materials, such as polyolefins or cellulose. Furthermore, the polyamides of the invention have superior strength and elongation values, good retention of physical properties above their glass transi- tion temperature, superior water resistance and barrier properties, low absorption of water, improved processability and mouldability, and excellent compatibility with polyolefins. The long chain aliphatic polyamides can be used for those applications in which polyolefins are typically used. However, polyamides provides in some benefits in that they are stronger and more wear resistant. Further, the polyamides of the invention and those prepared by the method of the invention display more resistance to acids than conventional polyamides.

DETAILED DESCRIPTION OF THE INVENTION

The polyamide of the invention is an AABB-type polyamide formed via the polymerisation of a diamine and a dicarboxylic acid where A and B refer to the -NH ₂ and -COOH groups on their respective diamine and dicarboxylic acid molecules. In general, polyamides are typically described as "PA X,Y" wherein X represents the number of carbon atoms derived from the diamine and Y represents the number of carbon atoms derived from the diacid. For example, PA 4,14 is a polymer consisting of a C4 diamine and C14 dicarboxylic acid.

An object of the invention is to provide a long chain polyamide (PA) having a number average molecular weight of at least 37,000 g-mol ^"1 and a general formula:

ΡΑ Χ,Υ

in which

X is an integer from 1 to 40, specifically 1 to 24, more specifically 4 to 18, still more specifically 4 to 6,

Y is an integer from 10 to 40, specifically 12 to 24, more specifically 14 to 18; comprising repeating units having formula I:

in which

R represents an aliphatic or aromatic, saturated or unsaturated hydro- carbyl moiety comprising one or more carbon chain lengths of C8 to C38, specifi cally C12 to C22, more specifically C12 to C16, optionally containing oxygen in its carbon chain;

R' represents an aliphatic or aromatic, saturated or unsaturated hy- drocarbyl moiety comprising one or more carbon chain lengths of CI to C40, spe- cifically CI to C24, more specifically C4 to C18, still more specifically C4 to C6, optionally containing oxygen in its carbon chain.

In another aspect, the invention provides a method for producing a long chain polyamide (PA) having a number average molecular weight of at least 37,000 g-mol ¹ and a general formula:

PA X,Y

in which

X is an integer from 1 to 40, specifically 1 to 24, more specifically 4 to 18, still more specifically 4 to 6,

Y is an integer from 10 to 40, specifically 12 to 24, more specifically 14 to 18; comprising repeating units having formula I:

in which

R represents an aliphatic or aromatic, saturated or unsaturated hydro- carbyl moiety comprising one or more carbon chain lengths of C8 to C38, specifically C12 to C22, more specifically C12 to C16, optionally containing oxygen in its carbon chain;

R' represents an aliphatic or aromatic, saturated or unsaturated hy- drocarbyl moiety comprising one or more carbon chain lengths of CI to C40, spe- cifically CI to C24, more specifically C4 to C18, still more specifically C4 to C6, optionally containing oxygen in its carbon chain,

comprising the steps of:

- providing an aliphatic or aromatic, saturated or unsaturated dicar- boxylic acid, having a carbon chain length of CIO to C40, specifically C12 to C24, more specifically C14 to C18, optionally containing oxygen in its carbon chain,

- providing an aliphatic or aromatic, saturated or unsaturated diamine having a carbon chain length of CI to C40, specifically CI to C24, more specifically C4 to C18, still more specifically C4 to C6, optionally containing oxygen in its carbon chain, - dissolving the dicarboxylic acid in a lower alcohol of CI to C4, such as ethanol, an organic solvent or a mixture of the organic solvent and water,

- mixing the alcoholic solution of the dicarboxylic acid with the diamine to form a nylon salt precipitate,

- polymerizing the nylon salt precipitate at a temperature above the melting point of the nylon salt to form polyamide,

- cooling the polyamide,

- recovering the polyamide.

The dicarboxylic acid used for producing a polyamide of the invention is selected from aliphatic (linear or branched) and aromatic (ring chains) dicarboxylic acids, optionally containing oxygen in their carbon chain. In an embodiment, the acid is aliphatic. The dicarboxylic acids can be either saturated (no double bonds) or unsaturated (double bonds present). In an embodiment, the acid is saturated. The carbon chain length of the dicarboxylic acids is in the range of CIO to C40. In another embodiment, the carbon chain length is in the range of C12 to C24. In a further embodiment, the carbon chain length is in the range of C14 to C18. In an embodiment, the dicarboxylic acid is aliphatic. In another embodiment, the dicarboxylic acid is aliphatic and saturated with a chain length of C14 to C18. In another embodiment, the dicarboxylic acid is polyethylene glycol diacid (polyfethylene glycol) bis(carboxymethyl) ether), having the formula

It is also possible to use carboxylic acid derivatives, such as acid esters or acid chlorides instead of carboxylic acids.

The diamine used for providing the polyamide of the invention is selected from aliphatic and aromatic diamines, optionally containing oxygen in their carbon chain. In an embodiment, the diamine is aliphatic. The diamine can be either saturated or unsaturated. In an embodiment, the diamine is saturated. The carbon chain length of the diamine is in the range of CI to C40. In an embodiment, the carbon chain length is CI to C24. In another embodiment, the carbon chain length is C4 to C18. In still a further embodiment, the carbon chain length is C4 to C6. In an embodiment, the diamine is aliphatic. In another embodiment, the diamine is aliphatic saturated diamine with a chain length of C4 to C6. In a further embodiment, the diamine is hexamethylene-l,6-diamine. In a still further embodiment, the polyamine is poly(ethylene glycol) diamine having the formula

In an embodiment, the polyamide of the invention is a polymer consisting of a dicarboxylic acid having a chain length of C14 to C18 and a diamine having a chain length of C4 to C12. In another embodiment, the polyamide is composed of an aliphatic saturated dicarboxylic acid of C14 to C18 and an aliphat- ic saturated diamine with a chain length C6.

In an embodiment, the polyamide of the invention is selected from a group comprising PA 4,13, PA 4,14, PA 4,15, PA 4,16, PA 4,17, PA 4,18, PA, 4,20, PA 4,24, PA 6,13, PA 6,14, PA 6,15, PA 6,16, PA 6,17, PA 6,18, PA 6,20, PA 6,24, PA 11,12, PA 11,13, PA 11,14, PA 11,15, PA 11,16, PA 11,17, PA 11,18, PA 11,20, PA 11,24, PA 12,6, PA 12,8, PA 12,10, PA12,12, PA 12,13, PA 12,14, PA 12,15, PA 12,16, PA 12,17, PA 12,18, PA 12,20, PA 12,24, PA 14,6, PA 14,8, PA 14,10, PA 14,12, PA 14,13, PA14,14, PA14,15, PA 14,16, PA 14,17, PA 14,18, PA 14,20, PA 14,24, PA 16,6, PA 16,8, PA 16,10, PA 16,12, PA 16,13, PA 16,14, PA 16,15, PA 16,16, PA 16,17, PA 16,18, PA 16,20, PA 16,24, PA 18,6, PA 18,8, PA 18,10, PA 18,12, PA 18,13, PA 18,14, PA 18,15, PA 18,16, PA 18,17, PA 18,18, PA 18,20, PA 18,24. In another embodiment, the polyamide is selected from a group comprising PA 6,14, PA 6,16, PA 6,18 and PA 12,16.

The dicarboxylic acids and/or diamines used in the preparation of the polyamides can originate from fossil or renewable sources. In an embodiment, the dicarboxylic acids and/or diamines are obtained from renewable sources. In an embodiment, the dicarboxylic acids are from renewable oils or fats, such as vegetable oils comprising rapeseed oil, canola oil, castor oil, soy bean oil, palm oil, palm kernel oil, corn oil, coconut oil, sun flower oil, camelina oil, jatropha oil, thistle oil, olive oil, sesame oil, peanut oil, shea nut oil, poppy seed oil, melon seed oil, kapok seed oil, tallow tee oil, jojoba oil, linseed oil, hempseed oil, cottonseed oil, tung oil, tall oil, algae oil, microbial oil or animal fats or fish fats or yellow grease or brown grease, or used cooking oil, or sludge palm oil or spent bleaching earth oil, or renewable fatty acids such as palm oil fatty acid distillate or tall oil fatty acid distillate, or renewable waste oils, fats or fatty acids regarded as wastes or residues. In an embodiment, these vegetable oils and fats are castor oil, soybean oil, palm oil, linseed oil, sunflower oil, rapeseed oil, coconut oil, corn oil, fish oil, tallow and cottonseed oil. In another embodiment of the invention the diacids and/or diamines are derived from carbohydrates of renewables sources, such as carbohydrates from lignocellulosic materials, starch crops or sugar crops. In yet another embodiment of the invention, the diacids and/or diamines are derived from lignocellulosic materials of renewable sources.

In an embodiment, the polyamide of the invention has a number average molecular weight of at least 50,000 g-mol ¹ . In another embodiment, the num- ber average molecular weight is at least 55,000 g-mol ^-1 .

In an embodiment, the polyamide of the invention is aliphatic.

In an embodiment, the polyamide of the invention is a homopolymer. In another embodiment, the polyamide of the invention is a copolymer. In still another embodiment, the polyamide is a copolymer comprising a repeating unit of formula I in which R is an aliphatic hydrocarbyl moiety having 4 carbon atoms.

According to another embodiment of the invention, the polyamide according to the invention is a co-polymer comprising aliphatic long chain monomers. According to yet another embodiment of the invention, the co-polymer comprises C6 aliphatic diacid monomers (such as adipic acid) and one of more of aliphatic long chain monomers of carbon chain lengths for CIO to C40, specifically C12 to C24, more specifically C14 to C18.

According to the invention, the polyamide is co-polymer in which at least 5% of repeating units are long chain monomers, according to another embodiment of the invention at least 10%, at least 20%, at least 30%, at least 40% of repeating units are long chain monomers and according to yet another embodiment of the invention at least 50% of the repeating units are long chain monomers.

In an embodiment, the melting point T _m of the polyamide of the invention is at most 220°C. In another embodiment, the melting point is at most 210°C. In a further embodiment, the melting point is at most 200°C. In a still further embodiment, the melting point is at most 195°C.

In an embodiment, the decomposition temperature Td of the polyamide of the invention is at least 480°C. In another embodiment, the decomposition temperature is at least 485°C. In a further embodiment, the decomposition tem- perature is at least 490°C. In an embodiment, the polyamide of the invention is insoluble in formic acid.

The method of the invention involves formation of a 'nylon salt' between the long chain dicarboxylic acid and diamine. The nylon salt formation is typically in a solvent such as in low CI to C4 alcohol, such as ethanol, an organic solvent or a mixture of the organic solvent and water. The content of the organic solvent of the mixture ranges from 5wt% up to 100 wt%. The nylon salt as a precipitate is then filtered and heated whereby the polymerisation of the nylon salt by polycondensation reaction proceeds giving water as a by-product.

In an embodiment, the nylon salt is purified by recrystallizing it from the lower alcohol whereby excess diacid and diamine as well as any potential contaminants are removed. This ensures an exact stoichiometric ratio of the monomers and guarantees high monomer purity. In an embodiment, the nylon salt, optionally purified, is filtered and added to the reaction chamber as a dry powder. This allows greater volumes of polyamide to be prepared per batch, while constant monitoring to ensure a precise monomer ratio is not necessary. The alcohol filtered off can be subsequently recycled and re-used for dissolution and washing.

In general, the polycondensation reaction involved in the method of the invention can be described as follows:

I ?0

In an embodiment, the diacid and/or diamine used in the method are derived from renewable sources as defined above.

In the method of the invention, the dicarboxylic acid is first dissolved in a solvent, such as an alcohol. The alcohol can be a lower CI to C4 alcohol. In an embodiment, the alcohol is ethanol. To enhance the dissolution of the acid, heat treatment can be applied. The concentration of the diacid in the solvent is in the range of 5 wt% to 25 wt%. In an embodiment, the concentration is about 10 wt%.

The dicarboxylic acid dissolved in a solvent is then mixed with the diamine whereby a precipitation, that is a nylon salt, is formed. The salt is then re- moved, e.g. by filtration. In an embodiment, the recovered salt is purified, e.g. by washing with a lower alcohol of CI to C4, such as ethanol. The purification provides a high amount of a desirable dimer complex, that is said nylon salt, whereby undesired excess monomers and contaminants are removed. The stoichiometric amount of the monomers is important to control the molecular weight of the polyamide. In an embodiment, the molar ratio of the diamine to diacid is about 1:1. Improper stoichiometric balance can lead to a low molecular weight polyamide after a short polymerization time and premature termination of the polycondensation reaction. Stoichiometry is controlled by preparing the nylon salt in a precise 1:1 ratio of diacid:diamine.

The nylon salt, optionally purified, is then subjected to a polymerizing step at a temperature above the melting temperature of the nylon salt. In an embodiment, this temperature is about 5°C to about 50°C above the melting temperature. In another embodiment, the polymerization is carried out at a temperature which is about 30°C above the melting temperature of the nylon salt. The polymerization reaction is typically carried out at a temperature range of about 200°C to about 250°C.

The polymerization time depends on the type of polyamide produced. Typically, it is at least 8 hours. In an embodiment, the polymerization time is at least 12 hours. In a further embodiment, the polymerization time is at least 24 hours.

Polymerisation can be conducted with or without catalysts. Suitable catalysts are, e.g. metal oxides and carbonates; strong acids; lead monoxide; ter- ephthalate esters; acid mixtures and titanium alkoxide or carboxylates.

During the polymerization, water is removed via vacuum.

After achieving the desired molecular weight of the polyamide, the polymerization reaction is terminated. Termination can be carried out, e.g., by cooling. The polymerization reaction can also be terminated by adjusting the concentration of the diamine and diacid so that one of the diamine and diacid is present in slight excess. The monomer present in a minor amount is consumed first and the monomer present in a major amount dominates the end of the polymer chains until no further polymerization is possible.

The polyamides of the invention show superior mechanical properties. Further, they are insoluble in formic acid.

Various characteristics of the sulphur-containing polyamides of the invention were measured. The analysis methods for each characteristic are described in more detail below.

In an embodiment, the polyamides of the invention and the polyamides prepared by the method of the invention have at least one of the following features: - water absorption in the range of 0.01% to 15%

- melting point T _m in the range of 50°C to 390°C

- Young's modulus in the range of 50 to 5000 MPa

- molecular weight M _n up to 350000 g-mol ¹

- tear strength in the range of 5 to 70 kN-m.

The polyamides of the invention and the polyamides prepared by the method of the invention are suitable for, but are not limited to, films and coatings, food packaging films, furniture, appliances, sports equipment, consumer goods, wire and cable, and automotive components. Furthermore, due to the long ali- phatic segments included in the polyamides, the polyamides have an increased processability and compatibility with polyolefins compared to conventional nylon 6,6.

In an aspect, the invention provides use of the polyamides of the invention or the polyamides prepared by the process of the invention for films and coatings, food packaging films, furniture, appliances, sports equipment, consumer goods, wire and cable, and automotive components.

The following examples are presented for further illustration of the invention without limiting the invention thereto.

The water absorption content of the polyamides prepared in the fol- lowing examples was measured as follows: The polyamide was soaked into distilled water for 4 days. After this, they were taken out and excess water from the surface of the samples was dried gently by tissue paper. The water absorption percentages were calculated by the ratio of the dried and wet samples.

The number average molecular weight (Mn) of the polyamides was measured using proton NMR analysis. All proton Nuclear Magnetic Resonance (Ή NMR) measurements were conducted by Bruker AVANCE 400 MHz spectrometer at room temperature in deuterated chloroform with 10 v/v% of trifluoroacetic anhydride (TFFA). The co-solvent was used due to poor solubility of long aliphatic chain polyamides in pure chloroform.

Size exclusion chromatography (SEC) analyses were performed at room temperature with a Waters 717plus Autosampler, Waters 515 HPLC pump, and a Waters 2414 refractive index (RI) detector. A set of two columns in series (HFIP-803 and HFIP-804 'Shodex' columns, Showa Denko Europe GmbH.) was utilised. Hexafluoroisopropanol (HFIP) with 5mM sodium trifluoroacetate (CF ₃ COONa) was used as eluent at 0.5 ml-mnr ¹ , and calibration was done against PMMA standards. All samples were prepared at 1 mg-ml ^-1 concentrations using the eluent solvent.

The melting point (T _m ), crystallinity temperature (T _c ) and decomposition temperature (Td) of the polyamides prepared in the Examples were recorded on TA Q2000 Modulated Temperature DSC at 20°C/min heating rate and in the temperature range from -90°C to 250°C. The thermal decomposition properties were determined by TA Q500 TGA at 20°C/min heating rate and in the temperature range from 30°C to 800°C. The glass transition temperature was measured using TA Q800 DMA.

The tensile test was performed on a polyamide film specimen (5.3 x 20 mm) with a thickness of 0.1 mm using Instron 4204 Universal Tensile Tester with a 100 N static load cell in 50% humidity. The tensile force was increased gradually at 5 mm/min rate on the sample specimens. Tensile analysis of polyamides at- three different temperatures, 30°C, 70°C and 100°C, was conducted using a TA Instruments Q800 Dynamic Mechanical Analyzer operating in tensile mode. A force rate of 3 N/min was applied to the test specimens.

Impact strength was measured with a Zwick Pendulum impact tester, utilising an impact energy of 1 J. Specimens with average dimensions ~80 x 10 x 5 mm ³ were prepared utilising heated press treatment, after which a 45° v- notch with 2 mm depth was cut. The results presented are the average of five reproducible repeats.

Tear strength analysis was conducted utilising a modified trouser test. Rectangular specimens 20 mm in length and 12.5 mm wide were mounted with the longer dimension parallel to the direction of extension. A 10 mm notch was cut from the center of the specimen to one end resulting in two legs which were secured at opposite ends of the tensile geometry. An extension rate of 10 mm-min ^-1 was used to deform the materials. The results are the average of 5 measurements.

Example 1: Preparation of polyamide 6,14

Tetradecanoic diacid was dissolved in absolute ethanol at approximately 70°C to obtain a 10 wt% clear transparent solution. 5 mol% excess of hex- amethylene-l,6-diamine (HMDA) in ethanol solution (10 wt%) was added drop- wise to the mixture of the diacid under stirring. A nylon salt precipitated approximately after 10 min. After the addition was completed, the reaction mixture was continuously stirred at 70°C for 30 min, following by 1 h at 0°C (ice bath). The ny- Ion salt thus obtained was filtered, and the filtrate was washed with ethanol. The nylon salt product was dried overnight in a vacuum oven at 60°C.

The nylon salt was charged into a stainless steel reactor at room temperature for polymerizing the nylon salt. The temperature was increased gradual- ly from room temperature to 30°C above the nylon salt's melting point, that is to 250°C, under a nitrogen purge. After reaching 250°C, approximately after 20 min, the nitrogen purge was stopped, all valves of the reactor were closed, and said temperature was maintained for 2 h by heating under pressure. Nitrogen purge was applied again for 1 h to remove the major amount of water. Finally, medium- high vacuum (less than 0.07 mbar) was applied to remove the remaining water. The overall reaction time was 24 h, whereby sufficient molecular weight polyamide 6,14 polymer was achieved. The polymer was soaked into liquid nitrogen to cool down and to prevent thermal degradation. The mass yield of the polyamide was 95.3%. Example 2. Preparation of polyamide 6,16

Polyamide 6,16 was prepared from hexadecanoic diacid and HMDA analogously to polyamide 6,14 described in Example 1 except that the reaction temperature was 230°C . The yield of the polyamide was 97.8%.

Example 3. Preparation of polyamide 6,18

Polyamide 6,18 was prepared from octadecanoic diacid and HMDA analogously to polyamide 6,14 described in Example 1 except that the reaction temperature was 220°C. The yield of the polyamide was 92.7%.

Example 4. Preparation of co-polyamide 6,6-6,18

Co-polyamide 6,6-6,18 was prepared from a combination of two nylon salts; that of polyamide 6,6 prepared from hexanedioic acid and HMDA, and that of polyamide 6,18, prepared from octadecanedioic acid and HMDA. Each nylon salt was prepared analogously as that described in Example 1. The polymerisation of co-polyamide 6,6-6,18 was analogous to polyamide 6,14 described in Example 1 except that the ratio of 6,6:6,18 nylon salts was 1:2 by weight and that the reaction temperature was 270°C. Example 5. Preparation of polyamide 12,16

Polyamide 12,16 was prepared from hexadecanoic diacid and dodeca- methylenediamine analogously to polyamide 6,14 described in Example 1 except that the reaction temperature was 200°C. The yield of the polyamide was 98.7%.

The water absorption ability of polyamides depends on the density degree of amide linkages on polymer chains. A low number of amide linkages leads to less moisture attraction. The water absorption abilities of the polyamides of the invention prepared in Examples 1-3 and 5, and that of commercial PA6,6 (reference) are shown in Table 1. The polyamides of the invention with long chain aliphatic segments (PA6,14, PA6,16, PA6,18) show a significantly lower water absorption ability, due to the increased presence of hydrophobic aliphatic chain segments.

Table 1

The molecular weight of the polymers and degree of polymerization is an important characteristic of a polymer material. In general, high average molecular weight of polymer exhibits good mechanical and physical characteristics. The number average molecular weights of the polyamides prepared in Examples 1-3 and 5, and that of commercial PA6,6 (reference) are shown in Table 2. The high molecular weights are attributed to high monomer purity and precise stoichiometric ratio between diamine:diacid, which were facilitated using the described nylon salt preparation and purification method. This prevents premature termination of the polycondensation reaction, encouraging chain growth and ul- timately higher molecular weights. Furthermore, this eliminates the need for subsequent reactive extrusion following the polycondensation reaction, which is commonly employed in commercial polyamide production in order to increase molecular weight. The bulk of the commercial PAs exhibited M _n values in the range of 12,000-31,000 g-mol ¹ , which is quite typical of commercial polyamide grades. The polyamides of the invention have a Mn range of 49,000-72,000 g-mol ¹ . This increased number average molecular weight can be attributed to the synergy of a number of factors. Firstly, the extended reaction times were used in the preparation of the PAs of the invention (≥ 20 h). This is much longer than conventional polycondensation times of 3-10 h used for commercial production of PA. Furthermore, effective water removal and mechanical agitation during the polycondensation reaction encourage chain growth and reduce the likelihood of chain scission (degradation) or reaction termination.

When comparing the M _w and PDI behaviour, the Pas of the invention displayed remarkably higher values on average than their commercial counterparts. This confirms that significant segments of very-high molecular weight polymer chains are present within the greater polymer network. These broad PDI and high M _w values indicate the reaction time, mixing and effectiveness of water (condensate) removal during polycondensation were appropriate and effective.

Results from NMR and SEC analyses are given in Table 2.

Table 2

* Commercial polyamide references

Thermal characteristics of the polyamides prepared in Examples 1-3 and 5, and that of commercial PA6/6 (reference) are shown in Table 3. Increasing the length of the dicarboxylic segments and molecular weight results in an increased likelihood of polymer chain entanglements and greater energy required for segmental motions and rotation about bonds. Subsequently, this results in an increase glass transition temperature (T _g ) for the polyamides prepared in Examples 1-3 and 5, which increases polymer stability and structural integrity, while also extending the application temperature range. The low melting points of the polyamides of the invention prepared in Examples 1-3 and 5 provide improved processability, such as extrusion and injection moulding, allowing for lower processing temperatures and less energy input during processing. In addition, the low melting points are beneficial for co-processing of the polyamides with low- thermal stability materials such as cellulose. Table 3 also shows that the long chain polyamides of the invention prepared in Examples 1-3 and 5 have higher degradation temperature (Td) than that of commercial PA6/6 (reference). The higher Td improves the processability, such as extrusion and injection moulding, of the polyamides by allowing wider processing temperature window.

A distinct double melting peak was observed for samples PA 6,12, PA 6,14 and PA 12,16. The presence of two melting peaks is explained by the melting of two morphological regions, forms I and II. Form I is relatively fixed in the thermal process, while the form II melting temperature varies with annealing conditions and can either appear above or below Form I. Form I dominates the crystallization while form II corresponds to recrystallization during heating. Above glass transition temperature, the amorphous regions reach a maximum degree of flexibility, after which they can be aligned and transformed into crystallites, which contribute towards the total crystallinity of the polymer. These re- crystallization peaks are also observed in other semi-crystalline polymers, for instance polypropylene. In some cases, only a single endotherm peak with a shoulder appears during melting process such as in PA 6,16 and PA 6,18. In these instances, the crystalline forms I and II may have similar structure, resulting in their melting peaks being close to each other; this often results in overlapping melting peaks.

Table 3

* Melvin I. Kohan, Nylon Plastics Handbook, Hanser Publishers, 1995. The results of tensile test performed on polyamides prepared in Examples 1-3 and 5, and that of commercial PA6/6 (reference) are shown in Table 4. The results show that the polyamides retained a degree of mechanical strength and integrity above the glass transition temperature. This can be attributed to the 5 combined influence of long chain segments within the polyamide structure, high molecular weight and the increased T _g values which were a consequence of the aforementioned factors (cf. Table 3).

Table 4

10 Table 5 shows the solubility of the polyamides prepared in Examples

1-3 and 5. The polyamides showed resistance to a range of common solvents (as indicated by the negative signs), dissolving only in specific solvent blends.

Table 5

*Melvin I. Kohan, Nylon Plastics Handbook, Hanser Publishers, 1995. The effect of the high number average molecular weight (Mn) to the mechanical properties of a polyamide were tested using two PA 6,16 molecules with different molecular weights. Polyamides were prepared according to the method of the present invention and they were polymerized for 15 hours and 24 hours, respectively. The mechanical properties measured for the polyamides are shown in Table 6. It can be seen that the longer polymerization time provides higher molecular weight as well as superior mechanical properties.

Table 6

Results from impact strength analysis are given in Table 7.

Table 7

* Commercial polyamide references

The results show that commercial polyamides displayed impact strength values in the range 13-29 kj-m ² . The long chain PA 6,18 displayed a sig- nificant increase in impact strength with a value of 83.4 kj-m ² . This enhancement in impact resistance is derived from both the increased molecular weight and molecular weight distribution of the sulphur-containing polyamides of the invention. Furthermore, the increased volume of chain entanglements between fractions of very high molecular weight PA contributes towards energy absorption upon im- pact. However, the increased ductility and elongation due to reduced interchain hydrogen bonding encourages dissipation of applied energy through various chain motions rather than breakage or failure. Results from tear strength analysis are given in Table 8.

Table 8

* Commercial polyamide references

Commercial polyamides generally displayed tear strength values in the range of 15-20 kN-m, PA 10,10 displaying a maximum tear strength values of 25 kN-m. The polyamides of the invention displayed a remarkable increase in tear strength, exhibiting values in the range of 25-50 kN-m. Tear resistance behaviour is dominated by various factors, including branching, crystallinity, molecular weight and molecular weight distribution. Since all commercial and polyamides of the invention were linear and yielded crystallinity values within a similar range, the influence of these factors can be considered minimal. Rather, the increased molecular weight and molecular weight distribution of the polyamides of the invention are evident variables. As molecular weight- and distribution are increased, two main phenomena occur; namely 1] increased likelihood of chain en- tanglement and 2] a slower time-scale of motion. This enables the polyamide chain to better resist deformation under increased loads. Furthermore, the increased ductility and elongation at break of the polyamides of the invention contributes towards increased resistance to tear by allowing dissipation of applied load through chain slippage and other motions, rather than breakage. This is en- couraged by the reduced number of amide linkages per repeat unit, which leads to a net reduction in the likelihood of interchain hydrogen bonding.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven- tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.