AU3097492A

AU3097492A - Method for productional thermostable chemical resistant plastics

Info

Publication number: AU3097492A
Application number: AU30974/92A
Authority: AU
Inventors: Roger Dahl; Anita Martinsen; Steinar Pedersen; Per Skjellaug
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 1991-12-23
Filing date: 1992-12-09
Publication date: 1993-07-28
Also published as: FI943023A; NO175596C; CA2126576A1; NO915081D0; CN1076204A; HU9401894D0; WO1993013161A1; NO175596B; JPH07502555A; HUT68290A; FI943023A0; TW222658B; NO915081L; RU94031163A; EP0618938A1

Description

Method for productional thermostable chemical resistant plastics

The invention concerns a procedure for the production of thermostable chemical-resistant plastic. Extremely high resistance to chemicals can be achieved if a strong, homogenous cross-linked network is introduced into the polymer. This type of polymer will be particularly suitable for applications where oil- resistance is important, for example in oil booms. It is also suitable for foils, tarpaulins, membranes and as a hard-wearing top layer for floor coverings. Cables, pipes and rainwear can also be mentioned as potential application areas.

In the case of PVC in particular, cross-linking has long been considered a good method of improving the mechanical properties of this plastic at high temperatures. Many attempts have therefore been made to find technical solutions to this problem. The introduction of cross-linking with the aid of so-called reactive plasticizers, such as difunctional and trifunctional acrylates or methacrylates, has had some commercial success. These reagents are added in quantities of 20-50 pph (pph = parts per hundred parts of polymer), and they are cross-linked by exposure to radiation or free radicals. The network that is formed is strong and heterogeneous, which leads to a considerably more brittle finished product. At the same time this technique can easily reduce the thermostability of the product because of the degrading effect of the radiation or the free radicals (see for example W. A. Salmon & L. D. Loan; J. Appl. Polym. Sci., 16, 671, (1972)).

A homopolymer of polyvinyl chloride (PVC) has a certain content of reactive groups, inasmuch as the carbon-chlorine bond is polar and may be attacked by nucleophilic compounds. In particular, thiolate ions have proved to be good nucleophiles (see for example. K. Mori & Y. Nakamura; J. Macromol. Sci. Chem. A12, (2), 209, (1978)).

In a number of articles and patents, Mori and Nakamura have described cross-linking of PVC with various types of dithiol triazines.

There are several reasons why the above method has not been a commercial success, despite its many attractive aspects, since the cross-linking in this case is triggered by heat, the reactivity of the system must be so good that the cross-linking can be done without breaking down the polymer. At the same time there are many processing techniques which require that the cross-linking is only done after the actual processing of the material is concluded. All this means that a very difficult balancing act is necessary to create a functional system. A further complicating factor is that the triazines which have the highest reactivity at the same time have a degrading effect on PVC. The method is therefore hardly suitable for producing thermostable chemical-resistant products.

Cross-linking of PVC with organic alkoxysilanes has been the object of great interest in recent years. Several patent applications describe this technique - examples that can be mentioned are DE 3719151, JP 55151019 and NO 890543. The methods described using alkoxysilanes have the disadvantage that the cross-linking requires the addition of water or water vapour. Two separate processes thus become necessary to obtain the desired product.

The object of the invention is therefore to produce a thermostable chemical-resistant halogenous plastic, specifically a polyvinyl chloride. To achieve greatly enhanced chemical- resistance in the case of PVC, it is necessary to introduce a cross-linked network into the product. To retain a thermostability comparable to that of a standard PVC it is necessary that the cross-linking reaction can be carried out concurrently with the working of the material without requiring very much extra heat.

These and other objects of the invention are achieved by the procedure described below, and the invention is characterized and defined by the accompanying patent claims.

The present invention concerns a procedure for producing a chemical-resistant halogenous plastic. Reactive groups are added to a halogenous polymer and cross-linked by reaction with a multifunctional organic compound under the influence of heat during or after the processing.

PVC is a preferred polymer, but the method is applicable to the production of cross-linked products from other halogenous polymers too. The invention also concerns a chemical-resistant cross-linked plastic which consists of 20-98% of copolymer of vinyl chloride and a glycidylic monomer, 0-80% of plasticizer, 0.05-10% of a multifunctional organic cross-linker, 0.1-10% of stabilizer and 0-3% of lubricant. The application of this plastic in oil and chemical-resistant products, especially foils, is also covered by the invention.

PVC is one of the great thermoplastics and is regarded as a "mature" product. All the same, there is a steady increase in the consumption of PVC, and new areas of application are still being developed. PVC is mainly produced by means of suspension, emulsion, microsuspension or mass polymerization. The bulk of the PVC produced is homopolymeric. The unique feature of PVC is that it can be mixed with a great number of organic solvents, and this means that products at all grades of plasticity can be manufactured.

The generally poor thermal stability of PVC is due to defect structures which arise in the polymer chain during polymerization. These defect structures can be allylic and tertiary chlorine groups as illustrated below.

normal structure

allylic chlorine

tertiary chlorine

One advantage of these defect sites is that the chlorine groups here are far more reactive than normal. This allows reactions with cross-linkers which are nucleophiles. Unfortunately, though, the number of possible reaction sites is far too small to achieve sufficient cross-linking.

It is therefore an essential part of the invention that a halogenous polymer is used which has had extra reactive groups added, which in reaction with suitable cross-linkers can produce a strong, homogenous network. It is an advantage if the extra reactive groups are already added during polymerization. The polymer can then in itself be regarded as reactive.

There are a number of relevant comonomers of vinyl chloride in this respect, and the choice of reactive group in the monomer can be made from, for example, epoxy (glycidyl), carboxylic acids, acid anhydrides, hydroxyl, amines, amides, isocyanates and silanes. Glycidylic acrylates or methacrylates have provedd particularly favourable. This is because they are easily copolymerized with vinyl chloride in emulsion or suspension polymerization, and they are quickly used up so that there is no enrichment of them in the monomer phase. In addition, the epo xy groups have a suitable reactivity in the conditions in which the cross-linking has to be done.

Copolymerization of vinyl chloride and a glycidylic acrylate or methacrylate (GA), e.g. glycidyl methacrylate, is as mentioned above an example of the way epoxy groups can be introduced into a halogenous polymer. Of the total monomer amount, GA should make up 0.05-10 weight %, preferably 05.-5 weight %. The comonomer can be added in a freely-selected way, but it is an advantage if an even distribution of reactive groups in the polymer can be achieved. The polymer can also be produced as a standard PVC with the above-mentioned techniques. Glycidyl acrylate and butyl glycidyl acrylate are also suitable monomers.

In principle the cross-linker can be any multifunctional compound which is coreactive with the polymer's reactive groups - for example acid anhydrides, carboxylic acids, amines, amides, mercaptans, thiol triazines, amidazoles. Special preferences are phthalic anhydride, maleic anhydride, succinic anhydride, succinic acid, malonic acid, oxalic acid, adipic acid, 2- dibutylamino-4,6-dithiol triazine, 2,4,6-trithiol triazine and 1,6-hexane diamine.

To increase or adapt the reactivity of the system, catalysts, e.g. of the tertiary amine or Lewis acid types, can be used. The cross-linker may also be another reactive polymer containing the above-mentioned groups. The type of cross-linker which will give the desired result will depend on the total reaction system and reaction conditions. The cross-linker may be added at an arbitrarily chosen point in time, also before the actual polymerizate dries.

The cross-linking take place when the copolymer produced is mixed with the cross-linker, and there is a direct reaction between these while the product is being processed. If the cross-linker is a difunctional amine the reactions can be illustrated follows :

O

\ " (

H₃C - C - C - O - CH₂ - CH - CH₂ H₂C - CH - CH₂ - O - C - C - CH₃

)

H₂N ( CH₂ ) ₆ NH₂

_O OH OH O

) " I I I ⁽

H₃C - C - C - O - CH₂ - CH - CH₂ - NH ( CH₂)₆ - NH - CH₂ - CH - CH₂ O - C -C - CH₃

) (

) = polymer chain

In this procedure, the disadvantage of poor thermostability when using the above-menticned triazines can aisc be overcome because the system is so reactive that gocd results car. be achieved at a lower working temperature and without the addition of activating metallic salts. The polymer is thus exposed to less heat stress and retains its thermostability.

One can study the chemical-resistance by exposing the finished products to attacks by various types of solvents. For PVC, cyclohexanone and tetrahydrofuran (THF) are good solvents which will completely dissolve the products. Acetone and diesel are other relevant solvents which will attack and swell PVC to a certain extent. For products which are to be used in contact with chemicals and solvents, the time aspect is also important. The product must retain its structure and its mechanical propertiess during storage. This means that additives and added plasticizers must not be allowed to migrate from the product to the ambient medium. Its has emerged that these product requirements can be met if cross-linking is introduced. The invention is described in more detail in the following examples. Foils of varying composition are produced from both paste PVC and S-PVC. Different types of cross-linkers were used. The production method for the foils and methods of testing them are described below. If not otherwise stated, quantities are given in pph (weight parts per hundred parts of polymer).

PRODUCTION OF FOIL.

1. Paste PVC

The paste is mixed in a Hobart mixer in accordance with the ingredients given in Tables 1-3. The paste is stroked on to release paper immediately after mixing and gelatinized/rolled and if necessary pressed as described in Tables 1-3.

2. S-PVC

The mixing is done in a small Papenmeier mixer. All solids are mixed together from the start at "low" agitation. The plasticizer (DOP) is added when the temperature has reached 65°C. The mixing then continues at "high" speed until the temperature reaches 110°C. The powder is then cooled down to 40°C.

These mixtures (225 g) are rolled into a foil 1 mm thick on the same day - the rolling specifications are given in Table 4. The foils are then aged with heat treatment as described in Table 4.

TESTING THE FOILS

1. Gel in THF.

Jetons of foil are punched out and transferred to tetrahydrofuran (THF). After about 24 hours the gel jeton is evaluated (gel (Yes)/non-gel (No)/partial) . The quantity of gel (%) is determined by a simplified meth od where the jeton is weighed before and after storage in THF (24 hours). The percentage of cross-linked gel is determined by the formula:

% Gel = Weight_after/Weight_before x 100%

Weight_after - weight of gel after drying (50°C, 5 hours)

Weight_before - weight of jeton.

2. Stress relaxation

The degree of cross-linking in the foils is also assessed by stress relaxation tests in a dynamic spectrometer (Rheometrics RDS 7700). The conditions are given in Tables 1-2. The values given as percentages are the ratio between the stress relaxation module initially and after 100 seconds measured at 8% constant deformation.

3. Storage stability in diesel.

The foils are stored in diesel for seven full days. Before and after storage in diesel the foils are analysed for tensile strength, elongation before breakage, cold-flex temperature and weight changes.

Maximum tensile strength and elongation before breakage are measured with a UTS 10 universal testing machine at a stretching speed of 50/min. The specimens conform to the specifications of ISO R 527 (Test Type 2.).

Weight change after storage in diesel was calculated on the basis of the weight of the foil before and after storage. Before the foil was weighed after storage, it was dried with absorbent paper and only weighed 24 hours later.

Cold-flex was done in accordance with ISO 458, Parts 1 and 2.

Test 1

Six mixtures of paste PVC with different compositions were made as shown in Table 1. One of the tests (A1) was done with PVC homopolymer, while in the others (A2-6) a copolymer of PVC and glycidyl methacrylate (GMA) was used. The percentage of GMA is based on the added quantity of VCM. Equal amounts of plasticizer, stabilizer and epoxidized soya oil were added to the six mixtures. Different quantities of cross-linker were added to three of the mixtures. The samples A1-4 were rolled for 3 min . at 175°C and then pressed at the same temperature for 3+1 min . Testing of these foils showed that the foils without added cros - linker (A1-A3) were fully dissolved in tetrahydrofuran. The foil with least cross-linker (A-4) and 1% of GMA was partially dissolved. Samples A5-6 both had 2% of GMA but different quantities of cross-linker, and were treated in the same way with gelatinization and pressing. It can be seen from the test results that these were resistant in tetrahydrofuran. Cross-linking was formed. However, the double quantity of cross-linker in A6 had little extra effect compared with A5.

The introduction of reactive groups into the PVC chains leads to a slight reduction in thermostability. The use of different cross-linkers also leads to great differences, but the homopolymer will always be rather better than the reactive samples. It is nevertheless possible to optimize the recipes with respect to thermostability, for example by the choice of cross- linker. The product is manufactured in process conditions which do not require extra heat, so the thermostability of the product is maintained. TABLE 1. Test with Zisnet DB as cross-linker

1) DINP = di-isononyl phthalate

2) Lankromark LZ616 = Ca/Zn stabilizer from Lankro.

3) ESO = Epoxidized soya oil

4) Zisnet DB = 2-dibutylamino-4, 6-dithiol triazine from Sankyo asei.

Test 2

Seven different mixtures were produced from paste PVC. These are shown in Table 2. Homopolymer was used in Test B-1, while in the others a copolymer of PVC and 2% of glycidyl methacrylate was used. The same amount of additives was used in all mixtures. The quantity of cross-linker was varied. All samples except B-6 were gelatinized for 7 minutes, while the gelatinizing temperature was varied from 170 to 190°C. The results show that the two samples B1-2 without cross-linker were both dissolved in tetrahydrofuran. No major effect can be observe from doubling the amount of cross- linker (see B-3 and B-6). The gelatinizing temperature has a positive effect on the result, as gelatinization at 190°C produces better cross-linking than treatment at 170°C. A longer gelatinizing time also affects the result, as can be seen by comparing the mixtures B-6 and B-7.

TABLE 2. Test with Zisnet F as cross-linker

1) Zisnet F = 2,4,6-trithiol triazine from Sankyo Kasei. Test 3

Three mixtures were made (C1-3) with a copolymer of PVC and 2% of lycidyl methacrylate. The mixtures C4-6 had a content of 3% of glycidyl methacrylate. The same quantities of additives were used in all mixtures while the quantity of cross-linker (phthalic anhydride) was varied from 1.7 to 4.4. A gelatinizing time of 15 minutes at 190°C was used for all samples. The results show that only the samples with the highest content of GMA produced good cross-linking, and that the best result was achieved with the highest content of cross-linker.

TABLE 3. Test with phthalic anhydride as cross-linker

Gel in THF (Y/N) partial partial partial Y Y Y

Rhsometrics (%) 43.1 43.9 42.9 45.8 48.1 49.3

(150°C/8%) Test 4

Five mixtures were made from S-PVC with 1% of glycidyl methacrylate. All mixtures had the same quantity of additives, while the quantity of cross-linker varied. The mixtures were rolled for five minutes at 160°C and three of the samples were additionally heat-treated for 24 hours at 120°C. The results show that a higher degree of cross-linking was achieved in the samples D-3 and D-5, which contained different quantities of cross- linker, but which were both heat-treated. It is also notable that it is possible to carry out rolling of the product without any great degree of cross-linking arising. This means that cross- linking can be done on the finished, formed product.

TABLE 4. S-PVC cross-linked with Zisnet F

1) DOP = dioctyl phthalate

2) LF3655 = Pb stabilizer from Akzo

3) AC316A = polyethylene wax from Allied Chemical Corp. Test 5

Five different mixtures, E1-5, were made, one with PVC homopolymer and four mixtures of PVC and 2 and 3% of glycidyl methacrylate respectively and with different cross-linkers (see Table 5). Specimens of the foils were tested before and after seven days' storage in diesel. The results show that the tensile strength is more or less unchanged in all samples, as expected at room temperature. Measurement of elongation before breakage shows that the samples with most gel (good cross-linking) retain their properties, while those without and with little gel are reduced in elongation before breakage. The cold-flex temperature is an expression of brittleness. The table shows that Sample E-1, which is not cross-linked, has been attacked by the solvent and has become brittler. The other samples have retained their flexibility. A weight reduction in the samples after storage in diesel may for example be due to loss of plasticizer. Samples E-4 and E-5, which have a high degree of cross-linking, exhibit no weight changes after storage in diesel.

TABLE 5. Test of cross-linked foil before and after storage in diesel

1) Foils with Zisnet DB were gelatinized at a lower temperature and for shorter periods to avoid burning the foils. Good results were still achieved. As shown in the examples, with this method one can achieve cross- linked products with good resistance to solvents, and production takes place in conditions which do not require extra heat. The thermostability of the product is therefore maintained.

In selecting process conditions/chemicals one can customize the process and one can choose to carry out the cross-linking either during or after working the material. The type of cross-linker chosen will be crucial for the degree of cross-linking in the product.

Claims

Patent claims

1. Procedure for the production of chemical-resistant halogen- ous plastics,

c h a r a c t e r i z e d i n t h a t

reactive groups are added to a halogenous polymer, and the polymer is cross-linked by reaction with a multifunctional organic compound which reacts with the polymer's reactive groups under the influence of heat during or after the process of working the material.

2. Procedure according to Claim 1,

c h a r a c t e r i z e d i n t h a t

the reactive groups are added to the halogenous polymer by copolymerization.

3. Procedure according to Claim 1,

c h a r a c t e r i z e d i n t h a t

the reactive groups used are epoxy, carboxylic acids, acid anhydrides, hydroxyls, amines, amides, isocyanates or silanes.

4. Procedure according to Claim 1,

c h a r a c t e r i z e d i n t h a t

the multifunctional organic cross-linker used is phthalic anhydride, maleic anhydride, succinic anhydride, succinic acid, malonic acid, oxalic acid, adipic acid, 2-dibutyl amino-4,6-dithiol triazine, 2,4,6-trithiol triazine, 1,6- hexane diamine or a polymer containing any of these groups.

5. Procedure according to Claim 1,

c h a r a c t e r i z e d i n t h a t

the halogenous polymer is produced by copolymerization of vinyl chloride and a glycidylic acrylate.

6. Procedure according to Claim 5,

ch a r a c t e r i z e d i n t h a t

the glycidylic acrylate used is glycidyl methacrylate, glycidyl acrylate or butyl glycidyl acrylate.

7. Procedure according to Claim 5,

c h a r a c t e r i z e d i n t h a t

a copolymer is used which contains 0.05-10 weight % of the glycidylic monomer.

8. Chemical-resistant cross-linked plastic,

c h a r a c t e r i z e d i n t h a t

the plastic consists of 20-98% of polymer of vinyl chloride and a glycidylic monomer, 0-80% of plasticizer, 0.05-10% of a multifunctional organic cross-linker, 0.1-10% of stabilizer and 0-3% of lubricant.

9. Chemical-resistant cross-linked plastic according to

Claim 8,

ch a r a c t e r i z e d i n t h a t

the multifunctional organic cross-linker is phthalic anhydride, maleic anhydride, succinic anhydride, succinicc acid, malonic acid, oxalic acid, adipic acid, 2- dibutylamino-4, 6-dithiol triazine, 2,4,6-trithiol triazine or 1,6-hexane diamine.

10. Chemical-resistant cross-linked plastic according to

Claim 8,

ch a r a c t e r i z e d i n th a t

the glycidylic acrylate is glycidyl methacrylate, glycidyl acrylate or butyl glycidyl acrylate in a quantity of 0.05-10 weight %.

11. Use of a plastic produced by known processing methods for S- PVC or paste PVC based on a recipe specifying 20-98% of copolymer of vinyl chloride and a glycidylic monomer, 0-80% of plasticizer, 0.05-10% of a multifunctional organic cross- linker, 0.1-10% of stabilizer and 0-3% of lubricant, and cross-linked subject to heat, as oil-resistant and chemical- resistant products.