def send_tokens(self, to: str, amount: int, erc20_address: str, private_key: str) -> bytes:
        """
        Send tokens to address
        :param to:
        :param amount:
        :param erc20_address:
        :param private_key:
        :return: tx_hash
        """
        erc20 = get_erc20_contract(self.w3, erc20_address)
        account = Account.privateKeyToAccount(private_key)
        tx = erc20.functions.transfer(to, amount).buildTransaction({'from': account.address})
        return self.ethereum_client.send_unsigned_transaction(tx, private_key=private_key)